fix(ui): iOS soft-keyboard shell alignment + hotseat relock on return

iOS Safari/WKWebView does not shrink the layout viewport for the soft
keyboard (Android Chrome does): the visual viewport shrinks AND offsets
down toward the focused field, and the values do not cleanly revert. The
pinned shell tracked only the height (--vvh), not the offset, so its
top-anchored content misaligned on iOS — empty space below the form,
worst on a repeat focus. Fix (one place; covers NewGame, Chat, Feedback):
- app.svelte.ts syncViewport: also mirror visualViewport.offsetTop into
  --vv-top, and scroll the focused field into view on the keyboard-open
  transition (iOS does not reliably scroll a pinned document to it).
- app.css: the pinned app-shell body follows top=--vv-top + height=--vvh
  (was inset:0), so it stays on the visible area.
- e2e/viewport.spec.ts emulates the visual-viewport resize+offset (a fake
  window.visualViewport) to verify the shell follows, on Chromium+WebKit.

Hotseat: returning to the lobby now re-locks the current seat (its PIN is
re-prompted) — LocalSource.relock, cleared in Game.svelte onDestroy.
Root-caused a crash it exposed: a $props() value (id) reads back undefined
during Svelte teardown, so isLocalGameId(id) threw and aborted onDestroy
(breaking navigation) — read the id from the loaded view instead.

ui/src/lib/localgame/source.ts

@@ -214,6 +214,14 @@ export class LocalSource implements GameLoopSource {
     return this.stateView(entry);
   }
 
+  /** relock drops any per-turn seat unlock, so re-opening a hotseat game re-prompts the current
+   *  seat's PIN. Called when the game screen is left (returning to the lobby). No-op when the game is
+   *  not cached, or a seat with no PIN (which is never withheld anyway). */
+  relock(gameId: string): void {
+    const entry = this.live.get(gameId);
+    if (entry) entry.unlockedSeat = null;
+  }
+
   /** verifyHostPin reports whether pin matches the hotseat master (host/referee) PIN. */
   async verifyHostPin(gameId: string, pin: string): Promise<boolean> {
     const entry = await this.load(gameId);