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>

screens/vr.h

@@ -17,7 +17,7 @@ struct ft_dmabuf {
     int fd[4];
 };
 
-enum ft_event_type { FT_MOTION, FT_BUTTON, FT_SCROLL, FT_LEAVE, FT_QUIT };
+enum ft_event_type { FT_MOTION, FT_BUTTON, FT_SCROLL, FT_LEAVE, FT_QUIT, FT_KEY, FT_KEYBOARD_CLOSED };
 
 struct ft_event {
     enum ft_event_type type;
@@ -26,6 +26,7 @@ struct ft_event {
     uint32_t button;  // FT_BUTTON: linux BTN_*
     bool pressed;
     double dx, dy;    // FT_SCROLL: notches (positive dy: scroll down)
+    uint32_t key;     // FT_KEY: linux KEY_* from our keyboard (pressed: down or up)
 };
 
 bool ft_vr_init(void);
@@ -44,6 +45,12 @@ bool ft_vr_screen_present(int index, const void *key, const struct ft_dmabuf *bu
 void ft_vr_forget(const void *key);
 // Poll panel input and SteamVR events.
 void ft_vr_poll(void (*handle)(const struct ft_event *, void *), void *data);
+// Our keyboard (keyboard.cpp), for typing on screen `index`: its keys arrive as FT_KEY
+// events, and FT_KEYBOARD_CLOSED when its Close key is pressed or the screens hide. False if
+// it can't be shown or there's no head pose. With the Steam menu or Steam's own keyboard
+// up, it waits and appears when they're gone.
+bool ft_vr_keyboard_show(int index);
+void ft_vr_keyboard_hide(void);
 // A command from the control socket (@ft_screens); writes the reply (see vr.cpp).
 void ft_vr_command(const char *command, char *reply, int reply_size);