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.

docs/TESTING.md

@@ -20,7 +20,18 @@ tests or touching CI.
   derivation and the GCG share/copy/download choice, plus Playwright specs against the
   mock for the friends screen (code issue/redeem, accept a request), the lobby
   invitations section, the stats screen, profile editing, and the export chooser's
-  finished-only visibility + its signed-URL download flow (route-intercepted).
+  finished-only visibility + its signed-URL download flow (route-intercepted). The
+  **offline mode** spec (`e2e/offline.spec.ts`) plays a full device-local `vs_ai` game
+  end-to-end: it forces the installed-PWA display mode, enters offline through the
+  Settings toggle (whose readiness check fetches the enabled variants' dawgs), then creates
+  and plays a local game with a **pinned bag seed** (`window.__mock.setLocalSeed`, so the
+  rack is deterministic and the human can tap out a precomputed opening), asserting the
+  robot's real reply and the IndexedDB replay after a reload. A local game needs a real
+  dictionary, so the mock's `fetchDict` serves the per-variant dawgs from the preview
+  build's `/e2edict/`, copied in by `scripts/e2e-dict.mjs` from `E2E_DICT_DIR` — the `ui`
+  CI job fetches the `scrabble-dictionary` release like the Go jobs; locally it defaults to
+  the sibling `scrabble-solver/dawg`. These dawgs are never committed and never enter the
+  production build.
 - **Render sidecar** — `renderer/` (Node + skia-canvas executing the shared
   `ui/src/lib/gameimage.ts`) carries a `node --test` smoke: the committed fixture (a
   real self-played 35-move game) must rasterize to a plausible PNG