Word Search mobile: focused viewport, theme placement, unique-per-size words

Per field feedback.
* Each day is now THREE distinct puzzles: the three sizes draw DISJOINT word
  slices from a date-shuffled pool (small/med/large = 6/9/13, sum 28 unique).
  Curated fallback themes expanded to 30 words each; LLM proposals accepted only
  if they supply >= 28 unique words, else fall back. No more repeats across sizes.
* Word Search is now a focused game screen on mobile (same as Daily Word): body
  scroll locked + footer hidden (generalized .playing-game), and the grid sizes
  to the largest square that fits between the theme and the palette (container
  query) — the whole puzzle is on screen, no page scroll.
* Theme placement: full "Today's theme · <name>" on the size-selection screen;
  just the theme name on the puzzle itself, saving vertical space for Large.
* cosy → cozy. 🇺🇸

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/app.css

@@ -71,6 +71,6 @@ button { font-family: inherit; cursor: pointer; }
    the page's scroll). The .playing-word class is toggled by /play and always
    removed on navigation via effect cleanup. Mobile only — desktop is unaffected. */
 @media (max-width: 720px) {
-  html.playing-word, html.playing-word body { overflow: hidden; }
-  html.playing-word footer.site { display: none; }
+  html.playing-game, html.playing-game body { overflow: hidden; }
+  html.playing-game footer.site { display: none; }
 }