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/screens/Lobby.svelte

@@ -160,6 +160,7 @@
                 <span class="who">{t('invitations.from', { name: inv.inviter.displayName })}</span>
                 <span class="sub">{t(variantKey[inv.variant] ?? 'new.english')}</span>
               {/if}
+              {#if !inv.multipleWordsPerTurn}<span class="sub">{t('game.oneWordRule')}</span>{/if}
             </span>
             <span class="acts">
               {#if inv.inviter.accountId === myId}