fix(decky): the shortcut liveness guard answered "alive" for every appId

`shortcutStillExists()` extracted the store method before calling it:

    const get = appStore?.GetAppOverviewByAppID;
    return get(appId) != null;

`GetAppOverviewByAppID` reads the store's own state (`this.m_mapApps`), so
the unbound call throws on the lost `this` — and the function's own
`catch { return true }` swallowed it. The guard therefore returned "still
exists" for EVERY appId. Not a stale-data bug: it never once answered no.

Everything downstream of it was consequently inert. A dangling appId — the
documented hazard this guard exists to catch, since the id outlives the
shortcut in Steam's CEF localStorage across a plugin reinstall — was never
dropped, so `ensureGamepadUiShortcut` always took the reuse branch and
`SetShortcut*`'d a dead id (silent no-ops). The visible library entry never
came back, `recreateShortcuts` reported success having done nothing (its
toast only checks for a non-null appId, and the dead one is non-null), and
"Open Punktfunk" ran `RunGame` on the dead id — Steam answers that with
"Game configuration unavailable".

Call it as a method so `this` survives, and guard the global with `typeof`
first: `appStore` is Steam-injected, and a bare reference to a missing one
is a ReferenceError that optional chaining does not prevent — which would
have landed in the same catch.

Verified against the live Deck that hit this: evaluated both versions over
its actual appIds, and where the old guard says alive/alive, the fixed one
says alive for the live stream shortcut and dead for the dangling UI id —
so the stale key now drops and the entry is recreated on the next mount.

clients/decky/src/steam.ts

@@ -70,9 +70,18 @@ declare const appStore:
  *  entry from a false "missing". A confident null means the shortcut was deleted → recreate. */
 function shortcutStillExists(appId: number): boolean {
   try {
-    const get = appStore?.GetAppOverviewByAppID;
-    if (!get) return true; // no way to verify — preserve the reuse path
-    return get(appId) != null;
+    // Call it as a METHOD on appStore — NEVER as an extracted function. Its implementation
+    // reads the store's own state (`this.m_mapApps`), so `const get = appStore.GetAppOverview…;
+    // get(id)` throws on the lost `this`, and the catch below turns that into a permanent
+    // "true". That is not a stale-data bug but a total one: the guard then answers "still
+    // exists" for EVERY appId, so a dangling id is never dropped, the reuse path repoints a
+    // dead shortcut (silent no-ops), and "recreate" reports success having done nothing.
+    // `typeof` first: `appStore` is a Steam-injected global, and a bare reference to a missing
+    // one is a ReferenceError that optional chaining does NOT prevent.
+    if (typeof appStore === "undefined" || !appStore?.GetAppOverviewByAppID) {
+      return true; // no way to verify — preserve the reuse path
+    }
+    return appStore.GetAppOverviewByAppID(appId) != null;
   } catch {
     return true;
   }