Word Search: pin per-size counts in test + fix stale .playing-word comment

Per Codex audit. test_wordsearch_endpoint now asserts the exact promise — small 6,
med 9, large 13, pairwise-disjoint. app.css comment updated to .playing-game (the
class was renamed when the focused viewport was generalized to both games).

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

frontend/src/app.css

@@ -66,10 +66,10 @@ button { font-family: inherit; cursor: pointer; }
 .rise { animation: rise 0.5s ease both; }
 @media (prefers-reduced-motion: reduce) { .rise { animation: none; } html { scroll-behavior: auto; } }
 
-/* Daily Word on mobile becomes a focused, full-viewport screen: lock body scroll
-   and hide the site footer so the on-screen keyboard is truly pinned (not riding
-   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. */
+/* A puzzle on mobile becomes a focused, full-viewport screen: lock body scroll and
+   hide the site footer so it feels like a game, not a web page (keyboard/grid
+   truly pinned, not riding the page scroll). The .playing-game class is toggled by
+   /play and always removed on navigation via effect cleanup. Mobile only. */
 @media (max-width: 720px) {
   html.playing-game, html.playing-game body { overflow: hidden; }
   html.playing-game footer.site { display: none; }