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/gamesource.ts

@@ -46,6 +46,9 @@ export const localSource = {
   unlockSeat: (id, pin) => load().then((s) => s.unlockSeat(id, pin)),
   verifyHostPin: (id, pin) => load().then((s) => s.verifyHostPin(id, pin)),
   hostAction: (id, pin, action, targetSeat) => load().then((s) => s.hostAction(id, pin, action, targetSeat)),
+  // relock is fire-and-forget: the engine is already loaded (the game was open), and re-locking a
+  // cached game before the screen is left needs no result.
+  relock: (id) => void load().then((s) => s.relock(id)),
   // events must return the unsubscribe synchronously (the screen subscribes in onMount), so it wires
   // the real subscription once the engine loads and, until then, cancels a pending subscribe.
   events: (id, onEvent) => {
@@ -60,7 +63,7 @@ export const localSource = {
     };
   },
 } satisfies GameLoopSource &
-  Pick<LocalSource, 'events' | 'create' | 'list' | 'delete' | 'unlockSeat' | 'verifyHostPin' | 'hostAction'>;
+  Pick<LocalSource, 'events' | 'create' | 'list' | 'delete' | 'unlockSeat' | 'verifyHostPin' | 'hostAction' | 'relock'>;
 
 /**
  * gameSource returns the source that runs the game with the given id: the local engine for a local