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/i18n/en.ts

@@ -69,6 +69,7 @@ export const en = {
   'game.dropGame': 'Drop game',
   'game.previewWords': '{words}: {n}',
   'game.previewIllegal': 'Not a legal move',
+  'game.oneWordRule': 'One word per turn',
   'game.chooseBlank': 'Choose a letter for the blank',
   'game.exchangeTitle': 'Select tiles to exchange',
   'game.exchangeConfirm': 'Exchange {n}',
@@ -233,6 +234,7 @@ export const en = {
   'new.moveTime': 'Move time',
   'new.hintsPerPlayer': 'Hints per player',
   'new.multipleWordsPerTurn': 'Multiple words per turn',
+  'new.start': 'Start game',
   'new.invited': 'Invitation sent.',
   'new.noFriends': 'Add friends first to invite them.',