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/codec.ts

@@ -238,6 +238,7 @@ function decodeGameView(g: fb.GameView): GameView {
     players: g.players(),
     toMove: g.toMove(),
     turnTimeoutSecs: g.turnTimeoutSecs(),
+    multipleWordsPerTurn: g.multipleWordsPerTurn(),
     moveCount: g.moveCount(),
     endReason: s(g.endReason()),
     lastActivityUnix: Number(g.lastActivityUnix()),
@@ -686,6 +687,7 @@ function decodeInvitationTable(i: fb.Invitation): Invitation {
     turnTimeoutSecs: i.turnTimeoutSecs(),
     hintsAllowed: i.hintsAllowed(),
     hintsPerPlayer: i.hintsPerPlayer(),
+    multipleWordsPerTurn: i.multipleWordsPerTurn(),
     dropoutTiles: s(i.dropoutTiles()),
     status: s(i.status()),
     gameId: s(i.gameId()),
@@ -721,6 +723,7 @@ function emptyGame(): GameView {
     players: 0,
     toMove: 0,
     turnTimeoutSecs: 0,
+    multipleWordsPerTurn: true,
     moveCount: 0,
     endReason: '',
     lastActivityUnix: 0,