test(offline): mock e2e for a device-local vs_ai game (C8)

A Playwright spec drives the whole offline flow in the mock build: force the
installed-PWA display mode, enter offline via the Settings toggle (its readiness check
fetches the dawgs), assert the blue chrome + online-games-hidden + Stats-disabled, then
create and play a device-local English vs_ai game with a pinned bag seed (deterministic
rack NEWYMAO) -- play the opening WAY across the centre, watch the robot reply with a
real move, and reload to confirm the IndexedDB replay.

Enabling infra (all e2e-only; nothing enters the production build):
- mock/client.ts fetchDict serves the per-variant dawgs from the preview build's
  /e2edict/ (was: threw 'unsupported in mock').
- scripts/e2e-dict.mjs copies the real dawgs into dist-e2e from E2E_DICT_DIR (the ui CI
  job fetches the scrabble-dictionary release like the Go jobs; local default: the
  sibling scrabble-solver/dawg); playwright.config runs it between build and preview.
- localgame/id.ts setForcedSeed + gateway.ts window.__mock.setLocalSeed: a mock-only
  seam to pin a local game's bag seed (tree-shaken from prod).
- ci.yaml ui job: fetch the dawgs + pass E2E_DICT_DIR to the e2e step.
- docs/TESTING.md: the offline e2e + the mock-dawg wiring.

Verified: check 0 / unit 482 / e2e 198 (both engines) / app entry 113.8/114.

ui/src/lib/mock/client.ts

@@ -166,9 +166,13 @@ export class MockGateway implements GatewayClient {
     // The mock never blocks; the blocked screen is driven directly via the mock-mode __block seam.
     return { blocked: false, permanent: false, until: '', reason: '' };
   }
-  async fetchDict(_variant: Variant, _version: string, _opts?: { signal?: AbortSignal; reload?: boolean }): Promise<ArrayBuffer> {
-    // No local dictionary in mock mode; the caller falls back to the mock evaluate.
-    throw new Error('fetchDict unsupported in mock');
+  async fetchDict(variant: Variant, _version: string, opts?: { signal?: AbortSignal; reload?: boolean }): Promise<ArrayBuffer> {
+    // The offline e2e serves the real per-variant dawgs from the preview build's /e2edict/ (copied
+    // in by scripts/e2e-dict.mjs from the scrabble-dictionary release, never committed), so a local
+    // vs_ai game can generate real moves. Version is ignored (a single served file per variant).
+    const res = await fetch(`/e2edict/${variant}.dawg`, { signal: opts?.signal });
+    if (!res.ok) throw new Error(`fetchDict ${variant}: HTTP ${res.status}`);
+    return res.arrayBuffer();
   }
 
   async reportLocalEval(_counts: Record<string, number>): Promise<void> {