Daily Word mobile: true viewport + flat warm keyboard + height-aware tiles

Make Daily Word feel like a focused mobile app screen, not a page with a keyboard.
* True viewport: while view==='play' && game==='word', a $effect locks body scroll
  and hides the site footer (mobile only), so the keyboard is genuinely pinned, not
  riding the document scroll. Effect cleanup ALWAYS removes the class on re-run or
  unmount, so leaving /play (back button OR any navigation) can never strand it.
* Keyboard restyled on-brand + modern: flat off-white (--surface) keys with a
  hairline border, soft 11px radius, no heavy raised shadow, ~46px tall, ↵ / ⌫
  glyphs, centered (max-width 430) instead of a full-bleed beige slab.
* Tiles now size to fit BOTH width and the height left above the keyboard
  (--tile = min(cap, width/cols, (100dvh-budget)/rows), gap 4px), so the active row
  and keyboard are always visible — Long Word's 6×7 gets slightly smaller tiles.

Real-device Safari/Chrome is the final check (100dvh + safe-area handling).

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

frontend/src/app.css

@@ -65,3 +65,12 @@ button { font-family: inherit; cursor: pointer; }
 @keyframes rise { from { opacity: 0; transform: translateY(6px); } to { opacity: 1; transform: none; } }
 .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. */
+@media (max-width: 720px) {
+  html.playing-word, html.playing-word body { overflow: hidden; }
+  html.playing-word footer.site { display: none; }
+}