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>

session/frametop-session.sh

@@ -21,11 +21,15 @@ here=$(dirname "$(readlink -f "$0")")
 # it should see the system as a normal login does, so drop the client's runtime: its
 # LD_LIBRARY_PATH put Steam's own libraries ahead of the system's (Steam's libavcodec has no
 # H.264 decoder, so VLC couldn't play most videos), and its overlay and launch settings are
-# meant for games. SteamOS's own defaults (/usr/share/deckard/mesavars.sh) stay.
+# meant for games. SteamOS's own defaults (/usr/share/deckard/mesavars.sh) stay. The
+# gamescope session also puts QT_IM_MODULE=xim and GTK_IM_MODULE=xim in the systemd user
+# environment, and with those, Qt and GTK apps never tell KWin a text field has focus, so
+# Frametop's keyboard (input/ft-textinput) never opens for them.
 for var in $(compgen -e); do
   case $var in
     LD_LIBRARY_PATH | LD_PRELOAD | STEAM_* | Steam* | SRT_* | PRESSURE_VESSEL_* | MANGOHUD_* | \
-      ENABLE_VK_LAYER_VALVE_steam_overlay_* | STEAMVIDEOTOKEN) unset "$var" ;;
+      ENABLE_VK_LAYER_VALVE_steam_overlay_* | STEAMVIDEOTOKEN | QT_IM_MODULE | GTK_IM_MODULE | \
+      XMODIFIERS) unset "$var" ;;
   esac
 done
 
@@ -133,9 +137,13 @@ fi
 
 # plasma-session starts KWin through kwin_wayland_wrapper. Shadow it to add our outputs.
 # With ft-screens the size is only the starting one: ft-screens sets each screen's own.
+# Our input method tells the input relay when a text field has focus, for SteamVR's
+# keyboard (input/ft-textinput).
+textinput=$(readlink -f "$here/../input/ft-textinput")
 cat > "$runtime/bin/kwin_wayland_wrapper" <<EOF
 #!/bin/sh
-exec /usr/bin/kwin_wayland_wrapper --width $width --height $height --output-count $screens --no-lockscreen "\$@"
+exec /usr/bin/kwin_wayland_wrapper --width $width --height $height --output-count $screens --no-lockscreen \\
+  --inputmethod $textinput "\$@"
 EOF
 chmod +x "$runtime/bin/kwin_wayland_wrapper"
 export PATH=$runtime/bin:$PATH