Keyboard patch cleanup; arrow icons centered again

- drop cruft: v4 DispatchKeypress undo, unused VKM import and
  __vrkbdPatched flag, redundant VRKBD_DISPLAY env in the unit
- one wrap() helper for all replaced functions; key names for the
  stylesheet derived from the key definitions instead of repeated
- unpatch: restore every layout ever patched (also ones disabled since)
  and the remembered component class instead of walking the tree again
- helper: debounce re-injects after bursts of new JS contexts; resolve
  pending DevTools calls when the connection closes
- arrow icons vertically centered again (hint stays at the bottom edge)
- updated header comments

modules/steam-keyboard-patch.nix

@@ -1,20 +1,16 @@
-# Esc/Ctrl/Alt and four arrow keys on Steam's VR keyboard.
-# The keyboard is part of Steam's UI (hardcoded layouts in steamui JS), and in
-# VR it can only send text; SteamClient.Input.ControllerKeyboardSetKeyState
-# throws "Unknown method". So:
-# - patch.js is injected at runtime into Steam's UI through its CEF
-#   DevTools port (127.0.0.1:8080; SteamOS starts Steam with
-#   -cef-enable-debugging). Steam's files are untouched, so updates don't undo
-#   it. It rebuilds the bottom row and, while Ctrl/Alt is active (or for Esc),
-#   hands the key combo to the helper instead of typing text. It also holds
-#   the real Ctrl/Alt down while the toggle is on (Ctrl+scroll), and routes
-#   characters Steam's key emulation turns into "1" (non-ASCII, AltGr/dead
-#   keys on the de keymap: |@{[]}\~^`äöü€…) to the helper.
-# - helper.mjs keeps that injection alive (Steam restarts, keyboard popup
-#   recreated) and presses the combos with `xdotool key` on :0, where gamescope
-#   keeps X focus on the window selected in VR. It only accepts ctrl/alt
-#   chords and the extra keys, so the Steam UI can't use it to type text or
-#   press Enter.
+# Runtime patch of Steam's on-screen (VR) keyboard. Steam's layouts are
+# hardcoded in its UI, and in VR it can only send text (no Ctrl/Alt/Esc;
+# SteamClient.Input.ControllerKeyboardSetKeyState throws "Unknown method").
+# - patch.js is injected into Steam's running UI through its CEF DevTools port
+#   (127.0.0.1:8080; SteamOS starts Steam with -cef-enable-debugging); Steam's
+#   files are untouched. Bottom row: Esc Ctrl Alt [space] AltGr ← ↑ ↓ →, with
+#   AltGr + arrows = Pos1/PgUp/PgDn/End. Chords, Shift+arrows and characters
+#   Steam's key emulation turns into "1" (non-ASCII, AltGr/dead keys on the
+#   de keymap) are handed to the helper; Ctrl/Alt are held while toggled.
+# - helper.mjs keeps the injection alive (Steam restarts, popup recreated),
+#   performs those requests with xdotool on :0 (X focus follows the window
+#   selected in VR) and reverts the patch when stopped (unpatch.js). Its
+#   allowlist can't type ASCII text or press Enter.
 # Depends on Steam UI internals; tested with Steam client 1790377368.
 { config, pkgs, lib, ... }: {
   imports = [ ./session.nix ];
@@ -39,7 +35,6 @@
           "${./steam-keyboard-patch/unpatch.js}"
           "${pkgs.xdotool}/bin/xdotool"
         ];
-        Environment = "VRKBD_DISPLAY=:0";
         Restart = "always";
         RestartSec = 5;
         TimeoutStopSec = 5;   # the helper unpatches Steam's UI on SIGTERM