feat(ui): single-word rule indicators + auto-match select redesign

Surface the per-game "single word" rule to the client and refine the
random-opponent New Game screen.

- Wire: thread multiple_words_per_turn into the GameView and Invitation
  FlatBuffers tables (Go + TS regenerated), through pkg/wire builders and both
  the backend push-event and gateway REST paths.
- In-game indicators (single-word games only): a small 1 in the status bar's
  score-preview slot (yields to the live preview) and a centred "One word per
  turn" label in the history-drawer header. Standard games show neither.
- Invitation card gains a "One word per turn" line for single-word invitations.
- Auto-match redesign: variant plaques are mutually-exclusive selects (highlight
  on tap, no longer enqueue); a lone offered variant is pre-selected; a bottom
  "Start game" button (disabled until a variant is chosen) confirms. The rule
  toggle appears once a Russian variant is selected.
- Tests: e2e for the new auto flow and the in-game indicator (mock g3 is a
  single-word game); mock/data + fixtures carry the new field. Docs: UI_DESIGN.

ui/src/lib/mock/client.ts

@@ -142,7 +142,7 @@ export class MockGateway implements GatewayClient {
   }
 
   // --- lobby ---
-  async lobbyEnqueue(variant: Variant, _multipleWords: boolean): Promise<MatchResult> {
+  async lobbyEnqueue(variant: Variant, multipleWords: boolean): Promise<MatchResult> {
     // Simulate a 10s-style robot substitution, sped up: match found shortly.
     const id = crypto.randomUUID();
     const g: MockGame = {
@@ -154,6 +154,7 @@ export class MockGateway implements GatewayClient {
         players: 2,
         toMove: 0,
         turnTimeoutSecs: 86400,
+        multipleWordsPerTurn: multipleWords,
         moveCount: 0,
         endReason: '',
         lastActivityUnix: Math.floor(Date.now() / 1000),
@@ -442,6 +443,7 @@ export class MockGateway implements GatewayClient {
       turnTimeoutSecs: settings.turnTimeoutSecs,
       hintsAllowed: settings.hintsAllowed,
       hintsPerPlayer: settings.hintsPerPlayer,
+      multipleWordsPerTurn: settings.multipleWordsPerTurn,
       dropoutTiles: settings.dropoutTiles,
       status: 'pending',
       gameId: '',