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/scripts/e2e-dict.mjs

@@ -0,0 +1,35 @@
+// Copies the real per-variant dictionary DAWGs into the mock e2e preview output (dist-e2e/e2edict/)
+// so the offline spec can create and play a real local vs_ai game (the mock's fetchDict serves these
+// files — see lib/mock/client.ts). The dawgs are NEVER committed and never enter the production
+// build; they live only in the throwaway dist-e2e/ that `vite preview` serves for Playwright.
+//
+// Source directory: E2E_DICT_DIR — in CI the fetched scrabble-dictionary release
+// ($GITHUB_WORKSPACE/dawg); locally it defaults to the sibling scrabble-solver/dawg checkout. A
+// missing source only warns (so the other, dawg-free specs still run); the offline spec then fails
+// with an obvious HTTP 404 from fetchDict.
+
+import { existsSync, mkdirSync, copyFileSync } from 'node:fs';
+import { join } from 'node:path';
+
+const srcDir = process.env.E2E_DICT_DIR ?? '../../scrabble-solver/dawg';
+const outDir = 'dist-e2e/e2edict';
+
+// The app's Variant enum value -> the release dawg file name (matches the movegen/parity mapping).
+const dawgFor = {
+  scrabble_en: 'en_sowpods',
+  scrabble_ru: 'ru_scrabble',
+  erudit_ru: 'ru_erudit',
+};
+
+mkdirSync(outDir, { recursive: true });
+let copied = 0;
+for (const [variant, file] of Object.entries(dawgFor)) {
+  const src = join(srcDir, `${file}.dawg`);
+  if (!existsSync(src)) {
+    console.warn(`e2e-dict: missing ${src} — the offline spec will 404 (set E2E_DICT_DIR)`);
+    continue;
+  }
+  copyFileSync(src, join(outDir, `${variant}.dawg`));
+  copied++;
+}
+console.log(`e2e-dict: copied ${copied}/${Object.keys(dawgFor).length} dawgs from ${srcDir} -> ${outDir}`);