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/screens/Lobby.svelte

@@ -78,7 +78,7 @@
 
   const variantKey: Record<string, MessageKey> = {
     english: 'new.english',
-    russian: 'new.russian',
+    russian_scrabble: 'new.russian',
     erudit: 'new.erudit',
   };
 </script>

ui/src/screens/Settings.svelte

@@ -10,6 +10,7 @@
   import { t, type Locale, type MessageKey } from '../lib/i18n/index.svelte';
   import type { ThemePref } from '../lib/theme';
   import type { BoardLabelMode } from '../lib/boardlabels';
+  import { insideTelegram } from '../lib/telegram';
 
   const themes: ThemePref[] = ['auto', 'light', 'dark'];
   const themeLabel: Record<ThemePref, MessageKey> = {
@@ -28,16 +29,18 @@
 
 <Screen title={t('settings.title')} back="/">
   <div class="page">
-    <section>
-      <h3>{t('settings.theme')}</h3>
-      <div class="seg">
-        {#each themes as th (th)}
-          <button class="opt" class:active={app.theme === th} onclick={() => setTheme(th)}>
-            {t(themeLabel[th])}
-          </button>
-        {/each}
-      </div>
-    </section>
+    {#if !insideTelegram()}
+      <section>
+        <h3>{t('settings.theme')}</h3>
+        <div class="seg">
+          {#each themes as th (th)}
+            <button class="opt" class:active={app.theme === th} onclick={() => setTheme(th)}>
+              {t(themeLabel[th])}
+            </button>
+          {/each}
+        </div>
+      </section>
+    {/if}
 
     <section>
       <h3>{t('settings.language')}</h3>