feat(offline): wire the local game source into the game screen (Phase B3.2)

The game screen now drives a local vs_ai game through the offline engine, dispatched by
game id — completing the playable local game (on top of the source, #193). Online play
is unchanged.

- gamesource.ts: gameSource(id) returns the local source for a `local:` id, else the
  gateway (the same game-loop interface). The offline engine stays OUT of the app entry
  bundle — it is dynamically imported on first use (a separate chunk), so online-only
  users never pay for it (the app entry stays within its size budget).
- localgame/id.ts: the tiny id helper (no engine imports) the dispatcher branches on.
- Game.svelte: the game-loop calls (state/history/submit/pass/exchange/resign/hint/
  evaluate/draft) go through gameSource(id) instead of the gateway directly; a local
  game's robot-reply events route through the same app event hub the network stream
  feeds, so the screen reacts to opponent_moved / game_over identically.

Behaviour-preserving for network games (gameSource returns the gateway for them). Local
verify green: check + test:unit + build + bundle-size gate + e2e (196 passed).

ui/src/game/Game.svelte

@@ -8,6 +8,7 @@
   import Board from './Board.svelte';
   import Rack from './Rack.svelte';
   import { gateway } from '../lib/gateway';
+  import { gameSource, localSource, isLocalGameId } from '../lib/gamesource';
   import { notePreviewLocal, notePreviewNetwork } from '../lib/localeval-metrics';
   import { navigate } from '../lib/router.svelte';
   import { app, handleError, showToast, markChatRead, seedChatUnread } from '../lib/app.svelte';
@@ -47,6 +48,13 @@
 
   let { id }: { id: string } = $props();
 
+  // The game's source: the local engine for an offline game id, otherwise the network gateway. The
+  // screen calls the game-loop methods (state/history/submit/pass/exchange/resign/hint/evaluate/
+  // draft) through it, so the same UI drives a local vs_ai game and an online one alike.
+  const source = $derived(gameSource(id));
+  // Unsubscribes from a local game's robot-reply events (offline only; null for a network game).
+  let localUnsub: (() => void) | null = null;
+
   let view = $state<StateView | null>(null);
   let moves = $state<MoveRecord[]>([]);
   let placement = $state<Placement>(newPlacement([]));
@@ -190,9 +198,9 @@
       // Fetch the saved draft alongside state and history (best-effort) so the composition is
       // applied in the same tick the board appears — never as a second, visible rack→board step.
       const [st, hist, draft] = await Promise.all([
-        gateway.gameState(id, includeAlphabet),
-        gateway.gameHistory(id),
-        gateway.draftGet(id).catch(() => ''),
+        source.gameState(id, includeAlphabet),
+        source.gameHistory(id),
+        source.draftGet(id).catch(() => ''),
       ]);
       view = st;
       syncWallet(st.walletBalance);
@@ -220,7 +228,7 @@
     setCachedDraft(id, json);
     if (draftSaveTimer) clearTimeout(draftSaveTimer);
     draftSaveTimer = setTimeout(() => {
-      void gateway.draftSave(id, json).catch(() => {});
+      void source.draftSave(id, json).catch(() => {});
     }, 500);
   }
   // applyDraft restores the player's saved composition over a freshly loaded state: the rack
@@ -264,6 +272,12 @@
         dict = m;
       });
     }
+    // A local (offline) game has no live stream: route the source's robot-reply events through the
+    // same app event hub the network stream feeds, so the event effect above reacts to
+    // opponent_moved / game_over identically.
+    if (isLocalGameId(id)) {
+      localUnsub = localSource.events(id, (e) => (app.lastEvent = e));
+    }
   });
 
   // Warm the game's dictionary for the local move preview once, when both the game and the
@@ -611,8 +625,9 @@
     // Flush a pending draft save so leaving mid-composition still persists it.
     if (draftSaveTimer) {
       clearTimeout(draftSaveTimer);
-      void gateway.draftSave(id, serializeDraft(rackIds, placement.pending)).catch(() => {});
+      void source.draftSave(id, serializeDraft(rackIds, placement.pending)).catch(() => {});
     }
+    localUnsub?.();
   });
 
   function onCell(row: number, col: number) {
@@ -740,7 +755,7 @@
     evalCtrl = ctrl;
     previewTimer = setTimeout(async () => {
       try {
-        preview = await gateway.evaluate(id, sub.tiles, variant, ctrl.signal);
+        preview = await source.evaluate(id, sub.tiles, variant, ctrl.signal);
         notePreviewNetwork();
       } catch {
         /* best-effort (or aborted) */
@@ -781,7 +796,7 @@
     if (!sub) return;
     busy = true;
     try {
-      applyMoveResult(await gateway.submitPlay(id, sub.tiles, variant));
+      applyMoveResult(await source.submitPlay(id, sub.tiles, variant));
       haptic('success');
       zoomed = false;
     } catch (e) {
@@ -800,7 +815,7 @@
   async function doPass() {
     busy = true;
     try {
-      applyMoveResult(await gateway.pass(id));
+      applyMoveResult(await source.pass(id));
     } catch (e) {
       handleError(e);
     } finally {
@@ -821,7 +836,7 @@
     resignOpen = false;
     busy = true;
     try {
-      applyMoveResult(await gateway.resign(id));
+      applyMoveResult(await source.resign(id));
       // Reveal the final board once the game is resigned: close the move-history drawer
       // (portrait — the landscape dock is unaffected) and zoom out if it was magnified.
       historyOpen = false;
@@ -834,7 +849,7 @@
   }
   async function doHint() {
     try {
-      const h = await gateway.hint(id);
+      const h = await source.hint(id);
       if (h.move.tiles.length && view) {
         placement = placementFromHint(h.move.tiles, view.rack);
         // Scroll the (zoomed) board to the hint's placement rather than the top-left:
@@ -906,7 +921,7 @@
     exchangeOpen = false;
     busy = true;
     try {
-      applyMoveResult(await gateway.exchange(id, tiles, variant));
+      applyMoveResult(await source.exchange(id, tiles, variant));
     } catch (e) {
       handleError(e);
     } finally {