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/lib/localgame/source.ts

@@ -13,6 +13,7 @@ import { serializeGame, replayGame, type LocalGameRecord, type Seat as LocalSeat
 import { getLocalGame, saveLocalGame } from './store';
 import { RULESETS } from './ruleset';
 import { getDawg } from '../dict';
+import { LOCAL_ID_PREFIX, isLocalGameId } from './id';
 import { decide } from '../robot/strategy';
 import { GatewayError, type PlacedTile } from '../client';
 import type {
@@ -31,13 +32,7 @@ import type {
 } from '../model';
 import type { Move } from '../dict/validate';
 
-/** LOCAL_ID_PREFIX marks a game id as a local (offline) game, so the app dispatches it here. */
-export const LOCAL_ID_PREFIX = 'local:';
-
-/** isLocalGameId reports whether an id belongs to a local game. */
-export function isLocalGameId(id: string): boolean {
-  return id.startsWith(LOCAL_ID_PREFIX);
-}
+export { LOCAL_ID_PREFIX, isLocalGameId };
 
 /** HINT_GATE_MS is the idle time since the robot's last move before an offline hint unlocks. */
 export const HINT_GATE_MS = 30 * 60 * 1000;