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/model.ts

@@ -38,6 +38,8 @@ export interface GameView {
   players: number;
   toMove: number;
   turnTimeoutSecs: number;
+  /** true = standard Scrabble; false = the single-word rule (Russian games). */
+  multipleWordsPerTurn: boolean;
   moveCount: number;
   endReason: string;
   /** Lobby sort key: the current turn's start (active) or the finish time (finished), Unix seconds. */
@@ -177,6 +179,8 @@ export interface Invitation {
   turnTimeoutSecs: number;
   hintsAllowed: boolean;
   hintsPerPlayer: number;
+  /** true = standard Scrabble; false = the single-word rule (Russian games). */
+  multipleWordsPerTurn: boolean;
   dropoutTiles: string;
   status: string;
   gameId: string;