feat(decky): full-featured Gaming-Mode client — fullscreen page, pairing, focus-correct launch

The plugin was a QAM launcher whose stream never appeared, with no
pairing. Three fixes, plus a headless --pair mode on the GTK client:

- Stream actually starts (MoonDeck's proven mechanism): gamescope only
  focuses the process tree Steam launched via reaper, so a flatpak
  spawned from the (root) backend is invisible. The frontend now
  registers ONE hidden non-Steam shortcut pointing at bin/punktfunkrun.sh,
  passes the host as the shortcut's Steam launch options, and starts it
  with SteamClient.Apps.RunGame — gamescope then fullscreen-focuses it.
  The wrapper execs `flatpak run io.unom.Punktfunk --connect <host>`.
- Fullscreen page: routerHook.addRoute("/punktfunk") — host list,
  per-host Pair/Stream, and a settings section (resolution/refresh/
  bitrate/gamepad/mic, written to client-gtk-settings.json).
- Pairing: a gamepad-navigable PIN keypad. The host shows the PIN; the
  backend runs the SPAKE2 ceremony headlessly via the client's new
  `--pair <PIN> --connect host` CLI mode (app.rs), persisting the host
  as paired so the stream then connects silently. Same flatpak =>
  shared identity store, verified live (ceremony against a real host).
- Backend (main.py): discover / pair / runner_info / get_settings /
  set_settings / kill_stream; uses DECKY_USER_HOME so paths resolve to
  the deck user's flatpak install regardless of the plugin's root flag.

CI (decky.yml) and the sideload packager now ship bin/punktfunkrun.sh.
The Steam-shortcut launch and headless-pairing env follow MoonDeck
exactly but need a Deck in Gaming Mode to fully confirm.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/decky/src/backend.ts

@@ -0,0 +1,45 @@
+// Bridge to the Python backend (main.py) + shared types.
+import { callable } from "@decky/api";
+
+export interface Host {
+  name: string;
+  host: string;
+  port: number;
+  pair: string; // "required" | "optional"
+  fp: string;
+}
+
+export interface PairResult {
+  ok: boolean;
+  fp?: string;
+  error?: string;
+}
+
+export interface RunnerInfo {
+  runner: string; // absolute path to bin/punktfunkrun.sh
+  app_id: string; // flatpak app id
+  exists: boolean;
+}
+
+export interface StreamSettings {
+  width: number; // 0 = native
+  height: number; // 0 = native
+  refresh_hz: number; // 0 = native
+  bitrate_kbps: number; // 0 = host default
+  gamepad: string; // "auto" | "xbox360" | "dualsense"
+  compositor: string; // "auto" | "kwin" | "wlroots" | "mutter" | "gamescope"
+  inhibit_shortcuts: boolean;
+  mic_enabled: boolean;
+}
+
+export const discover = callable<[], Host[]>("discover");
+export const pair = callable<
+  [host: string, port: number, pin: string, name: string],
+  PairResult
+>("pair");
+export const runnerInfo = callable<[], RunnerInfo>("runner_info");
+export const getSettings = callable<[], StreamSettings>("get_settings");
+export const setSettings = callable<[settings: StreamSettings], { ok: boolean }>(
+  "set_settings",
+);
+export const killStream = callable<[], { ok: boolean }>("kill_stream");