Stage 17: UI defect fixes (russian variant, Telegram theme/nav/banner, reconnect, hint zoom, plaque, history, transitions, per-game cache)

- #6 align the UI variant id to the backend canonical 'russian_scrabble' (type, variants, Lobby, mock, tests) — fixes the New->Russian 400
- #11/#12 inside Telegram force the colour scheme from WebApp.colorScheme (over OS prefers-color-scheme, fixing the Telegram Desktop breakage) and hide the theme switcher
- #14/#15 nav bar takes Telegram's bg; announcement banner gets a dedicated subtle --ad-bg accent token
- #16 suppress the reconnect banner while backgrounded and silently reconnect the live stream on return to the foreground
- #17 hint zoom scrolls to the placement's bounding box, not the top-left
- #19/#20 players plaque: active seat raised with side shadows, others sunk; tap toggles history
- #21/#23 history: scrollbar-gutter:stable (no word jitter); fixed-height drawer pins the bottom shadow to the board
- #3 (UI) disable nudge on the player's own turn
- #18a directional screen slide transitions (forward in from the right, back reveals the lobby)
- #13 per-game in-memory cache: instant render on re-entry + background refresh
- e2e: openGame waits for the slide transition to settle

ui/src/game/Chat.svelte

@@ -6,12 +6,16 @@
     messages,
     myId,
     busy,
+    canNudge = true,
     onsend,
     onnudge,
   }: {
     messages: ChatMessage[];
     myId: string;
     busy: boolean;
+    // Nudging only makes sense while waiting on the opponent; it is disabled on the
+    // player's own turn (there is no one to hurry along).
+    canNudge?: boolean;
     onsend: (text: string) => void;
     onnudge: () => void;
   } = $props();
@@ -47,7 +51,7 @@
       onkeydown={(e) => e.key === 'Enter' && send()}
     />
     <button class="iconbtn" onclick={send} disabled={busy} aria-label={t('chat.send')}>⬆️</button>
-    <button class="iconbtn" onclick={onnudge} disabled={busy} aria-label={t('chat.nudge')}>🛎️</button>
+    <button class="iconbtn" onclick={onnudge} disabled={busy || !canNudge} aria-label={t('chat.nudge')}>🛎️</button>
   </div>
 </div>