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/game/Game.svelte

@@ -790,6 +790,7 @@
             {:else}
               <button class="hicon" onclick={() => (resignOpen = true)} aria-label={t('game.dropGame')}>🏁</button>
             {/if}
+            {#if !view.game.multipleWordsPerTurn}<span class="oneword-label">{t('game.oneWordRule')}</span>{/if}
             <button class="hicon" onclick={() => navigate(`/game/${id}/chat`)} aria-label={t('game.chat')}>
               💬{#if (app.chatUnread[id] ?? 0) > 0}<span class="cbadge">{app.chatUnread[id]}</span>{/if}
             </button>
@@ -857,7 +858,7 @@
         <span class="turn-ind">{isMyTurn ? t('game.yourTurn') : view.game.seats[view.game.toMove]?.displayName ?? ''}</span>
       {/if}
       <span class="scores">
-        {#if preview}{preview.legal ? t('game.previewWords', { words: preview.words.join(', '), n: preview.score }) : t('game.previewIllegal')}{/if}
+        {#if preview}{preview.legal ? t('game.previewWords', { words: preview.words.join(', '), n: preview.score }) : t('game.previewIllegal')}{:else if !view.game.multipleWordsPerTurn}<span class="oneword" title={t('game.oneWordRule')}>1️⃣</span>{/if}
       </span>
     </div>
 
@@ -1103,6 +1104,18 @@
     min-width: 64px;
     text-align: right;
   }
+  .oneword {
+    font-size: 0.95rem;
+  }
+  /* The single-word-rule label centred in the history header between its two icons. */
+  .oneword-label {
+    flex: 1;
+    text-align: center;
+    font-size: 0.78rem;
+    font-weight: 600;
+    color: var(--text-muted);
+    white-space: nowrap;
+  }
   .rack-row {
     display: flex;
     flex: none;