Add a keyboard for text fields on the desktop

Fixes #7: SteamVR's keyboard never came up for the desktop's apps, and
opening it for our panels doesn't work well on the Frame (it's Steam's own
panel, mounted in the dashboard's scene, it follows the laser between
panels, and it takes the controllers over to SteamVR's laser).

- KWin starts input/ft-textinput as the desktop's input method. It tells
  the input relay when a text field gains or loses focus, and the relay
  asks ft-screens to open or close the keyboard. The session drops the
  QT_IM_MODULE=xim and GTK_IM_MODULE=xim that the gamescope session sets,
  or Qt and GTK apps never report text fields.
- The keyboard is ft-screens' own panel (screens/keyboard.cpp): a US laptop
  layout, typed with a controller's laser or the 3D mouse. It opens 0.7 m
  in front of you, below your eyes and facing you. It has a grab bar to
  move it, a Close key, latching Shift, Ctrl and Alt, and repeat on a held
  key. It's drawn into shared DMA-BUFs, so it doesn't flicker. Its keys
  reach the focused screen as key presses, so every app takes them.
- It steps aside while the Steam menu or Steam's own keyboard is up and
  comes back after. A layout reset closes it, and it doesn't open without
  a head pose.
- Frametop Input Settings has a Keyboard page: open it for every text
  field, only while no keyboard is connected (the default), only from a
  mapped button (the new Open/close keyboard action, for mice and
  controllers), or never. A switch keeps it open until you close it.
- The pointer helper treats every frametop.* overlay as a real panel. The
  keyboard's shared texture reports 0x0 like SteamVR's scene-graph
  controls, and the helper had given it their wide catch radius.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/reference.md

@@ -55,6 +55,8 @@ In the last three modes the hotkey shows the screens anyway. Two more settings o
 
 Input from the lasers reaches KWin through ft-screens' own seat. Keys come from the input relay, from pass-through keyboards and any key a pointer device passes through. Typing follows your last click: after a click on a screen it goes to the desktop, even with the SteamVR dashboard open, and after a mouse click on any other panel (the dashboard, Steam, an app like Spotify) it goes there instead. While it goes to the desktop, the relay grabs pass-through keyboards so gamescope, which reads every keyboard itself, doesn't type them into the Steam app too. A program that watches every keyboard for a hotkey loses a grabbed one; with `SHARE_KEYS=1` in `~/.config/frametop.conf`, their keys also go to `@frametop_keys` for it. That's off by default, since any local process that binds the name first would get everything typed into the desktop. Hidden screens don't take typing.
 
+Frametop's keyboard opens by itself when a text field on the desktop gets focus, and stays open until its Close key, a layout reset, or a mapped button closes it (or, with Keep it open off in Frametop Input Settings, until the text field loses focus). While the Steam menu (the dashboard) or Steam's own keyboard is up, it steps aside, and it comes back where it was when they're gone; one asked for meanwhile appears then. In the "only with the dashboard" visibility mode, the dashboard doesn't count. It doesn't open without a head pose (the headset in standby). It's a panel of keys (a US laptop layout, with Esc where Caps Lock would be, arrows, and a Close key) that ft-screens shows 0.7 m in front of you and below your eyes, facing you. It stays where it opened, and its grab bar (the pill along the top) moves it like a screen's. Type on it with a controller's laser or the 3D mouse. Shift, Ctrl and Alt latch for the next key, and a held key repeats. KWin starts `input/ft-textinput` as the desktop's input method, and KWin activates it whenever the focused app turns on text input for a field. It tells the relay (`textfield 1` or `0`), the relay decides by the Keyboard setting in Frametop Input Settings, and ft-screens opens the keyboard for the screen that has keyboard focus (`vrkeyboard show`, `hide`, or `toggle` from a mapped button). Its keys reach the focused screen as key presses, so it works in every app, but only apps that use Wayland text input (Qt, GTK, Firefox) open it by themselves; Chromium, Electron and X11 apps need the button. The session drops the `QT_IM_MODULE=xim` and `GTK_IM_MODULE=xim` that the gamescope session sets, or Qt and GTK apps wouldn't use Wayland text input either.
+
 KWin's nested backend doesn't undo a screen's scale on pointer input, so ft-screens divides panel positions (in pixels) by it. `ft-layout` sends it each screen's scale as KWin reports it (`scale N s`) whenever it applies scales: at desktop start and from Frametop Display Settings. A scale changed only in Plasma's own display settings is put back to the Frametop layout's the next time `ft-layout` runs.
 
 ft-screens listens for datagrams on the abstract socket `@ft_screens` and replies to the sender:
@@ -62,7 +64,7 @@ ft-screens listens for datagrams on the abstract socket `@ft_screens` and replie
 ```
 place N x y z yaw pitch roll     width N metres          curve N radius|on|off
 pin N|all left|right|head [matrix]    unpin N|all        size N w h
-get N    screens    head    state    key code value    scale N s
+get N    screens    head    state    key code value    scale N s    vrkeyboard show|hide|toggle
 visibility always|dashboard|gesture|toggle    wrist degrees    gesture left|right degrees
 hide | show | toggle    controllers always|outside_games|dashboard    ingames hide|visible
 ```
@@ -106,11 +108,12 @@ The pointer settings are in `~/.config/frametop.conf`: `POINTER_SENSITIVITY`, `P
 
 ## Frametop Input Settings
 
-A Kirigami app with a Python backend, in the Plasma menu under Settings. It runs in the `dev` container and talks to the relay over its control socket, `@frametop_relay`. It has seven pages:
+A Kirigami app with a Python backend, in the Plasma menu under Settings. It runs in the `dev` container and talks to the relay over its control socket, `@frametop_relay`. It has eight pages:
 
 - Devices lists every USB and Bluetooth mouse and keyboard, with a light that flashes when the device is used. Each device gets a role: 3D pointer (grabbed, drives the pointer; the default for anything with a mouse), Pass through (grabbed only while typing goes to the desktop; the default for keyboards, where a Meta tap toggles the dashboard if `META_DASHBOARD=1` is in `~/.config/frametop.conf`), or Ignore. A device is identified by its Bluetooth address, or its USB ids and name, so all of its input nodes share one role. Forget drops everything saved for a device.
-- Buttons maps a pointer device's buttons. Choose Capture a button, press the button or key, then pick an action: a click, back, scroll, toggle dashboard, recenter, pointer on or off, head follow on or off, gaze pointer on or off, faster or slower, pass the key through, or nothing. Devices with saved mappings are listed even while they're asleep.
+- Buttons maps a pointer device's buttons. Choose Capture a button, press the button or key, then pick an action: a click, back, scroll, toggle dashboard, recenter, pointer on or off, open or close the keyboard, head follow on or off, gaze pointer on or off, faster or slower, pass the key through, or nothing. Devices with saved mappings are listed even while they're asleep.
 - Controllers maps the Frame controllers' buttons (every button but the system button) to the same actions, except passing a key through. Capture a button and press it on a controller, or pick it from the list. The controllers aren't input devices on the host; only SteamVR sees them. So the pointer helper reads them with SteamVR input (`pointer/helper/vrbuttons.h`, `pointer/helper/actions/`) and sends presses to the relay (`vrbtn right/a 1`), which does the mapped action. The helper only takes the buttons that are mapped (the relay tells it with `vrbind`), at an overlay-global priority, and only while no game (scene application) runs, so games keep every button; with In games on (`controller_in_games`), a mapped button is taken from games too. That needs SteamVR's "Enable global input from overlays (Experimental)" setting (`steamvr/globalActionSetPriority`), which the page's Global input switch turns on and off. Mappings are saved as `controller_buttons` in `~/.config/frametop-input.json`.
+- Keyboard sets when Frametop's keyboard opens: whenever a text field is selected; only while no pass-through keyboard is connected (the default; keyboards other programs make through uinput, like frame-voice's, don't count); only with a mouse or controller button mapped to Open/close keyboard; or never, which turns the button off too. Keep it open (on by default, `vr_keyboard_persist`) leaves it open after the text field loses focus. The mode is saved as `vr_keyboard` in `~/.config/frametop-input.json`, and the page lists the keyboards that count as connected.
 - Pointer has a Head follow switch and sliders for the pointer settings, which apply immediately, and a Recenter button.
 - Ignored panels lists the SteamVR overlays that are showing, grouped by app (the first two parts of the overlay key, such as `sasaken.frame-perf-overlay`), from the pointer helper (`overlays`). Tick a panel, or Ignore the whole app, and the pointer passes through it to what's behind. It's for panels you only look at, like a performance overlay that follows your view. The list is saved as `POINTER_IGNORE` in `~/.config/frametop.conf`: comma-separated overlay keys, where a shell pattern like `vendor.app*` covers a whole app, including panels it opens later. The helper reloads at once. Frametop's own screens aren't listed, and entries for apps that aren't open are listed below, to remove.
 - Gaze has the gaze pointer switch (on now and from now on; a mapped button toggles it until the helper restarts), the gaze mode sliders, the gaze service's state (headset, samples per second, how often the tracker is losing each eye, the calibration, the nudges learned), and Calibrate (opens the gaze probe), Check headset fit (opens the probe's Headset fit mode), Reload calibration, and Forget nudges.