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.

docs/UI_DESIGN.md

@@ -108,7 +108,10 @@ Login uses `Screen`.
   longer merely scrolls the board out of view, which used to leave a stale-open state that
   made a follow-up plaque tap "jump" the board). The drawer carries its own **header**: a 🏁
   **Drop game** (or 📤 **Export GCG** on a finished game) at the left and the comms 💬
-  (badged with unread chat) at the right, icon-only. Each **opponent**'s card also gains a
+  (badged with unread chat) at the right, icon-only. A **single-word-rule** game (a Russian
+  game with "multiple words per turn" off) centres a **"One word per turn"** label between
+  those two icons, and the status bar shows a small **1️⃣** in the score-preview slot that
+  yields to the live word/score preview while tiles are pending; standard games show neither. Each **opponent**'s card also gains a
   🤝 **add-friend** control (non-guests; hidden once a friend, disabled once requested) that
   confirms via the fading-✅ tap, swapping the card's score for "Add friend?" while armed
   (see Controls); the name and score stay centred — the 🤝 is pinned to the card's edge.
@@ -180,11 +183,15 @@ IV 🏅; active games show Your move 🟢 / Opponent's move ⏳; invitations use
   list (Remove / Block), and a **blocked** list (Unblock). Durable accounts only — a
   guest sees a sign-in prompt.
 - **Invitations**: a lobby **section** (a 💌 row per open invitation) with Accept /
-  Decline for an invitee and a waiting/Cancel state for the inviter; creating one is the
-  **"Play with friends"** mode in `NewGame.svelte` (pick invitees, then variant / move
-  time / hints). For a **Russian** variant (auto-match or invite) a **"Multiple words per
-  turn"** checkbox (`.toggle`, **default off** = the single-word rule) appears; English
-  variants never show it.
+  Decline for an invitee and a waiting/Cancel state for the inviter; a single-word-rule
+  invitation adds a **"One word per turn"** line to the card. Creating a game lives in
+  `NewGame.svelte`: **"Play with friends"** (pick invitees, then variant / move time /
+  hints) and **auto-match** (random opponent). The auto-match variant plaques are
+  **mutually-exclusive selects** — a tap **highlights** one (an accent inset border) instead
+  of starting a game; a lone offered variant is pre-selected, and a bottom **Start game**
+  button (disabled until a variant is chosen) confirms. For a **Russian** variant (either
+  flow) a **"Multiple words per turn"** checkbox (`.toggle`, **default off** = the single-word
+  rule) appears once that variant is selected; English variants never show it.
 - **Statistics** (`screens/Stats.svelte`, the lobby 📊 tab): a 2-column grid of stat
   cards (wins / losses / draws / games / win-rate / best game / best move) — pure
   numbers, no charts.