Stage 7 polish: game rework + board zoom + tests (Parts D/E/F/I)

- Board: fixed-viewport transform-scale zoom (animated) with counter-scaled cqw labels, corner letters, bonus-label modes (boardlabels), contrasting grid lines
- Game: Screen shell + game tab-bar (Draw/Skip/Hint/Shuffle) via HoldConfirm popovers; MakeMove 🏁 + compact popup; rack collapses used slots; hint places tiles on board (placementFromHint) + no_hint_available toast; Scores:N replaces Hints; history slide-down (swipe/click, scroll-locked); check-word alphabet/length limit + in-memory cache + 5s throttle
- backend: no_hint_available result code split + test
- vitest: banner rotator + linkify, resultBadge, boardlabels, placementFromHint (29 tests); Playwright smoke updated; prod bundle ~74 KB gzip

ui/src/app.css

@@ -23,7 +23,7 @@
   /* board + tiles (all drawn with CSS primitives) */
   --board-bg: #cdd6cf;
   --cell-bg: #e7ece8;
-  --cell-line: #b6c0b8;
+  --cell-line: #7f8d83;
   --tile-bg: #f4e2b8;
   --tile-edge: #d8c190;
   --tile-text: #2a2113;
@@ -62,7 +62,7 @@
 
     --board-bg: #2a3330;
     --cell-bg: #222a27;
-    --cell-line: #38433d;
+    --cell-line: #56655c;
     --tile-bg: #d9c79a;
     --tile-edge: #b6a473;
     --tile-text: #20190d;
@@ -136,6 +136,8 @@ button {
   font: inherit;
   color: inherit;
   cursor: pointer;
+  user-select: none;
+  -webkit-user-select: none;
 }
 
 .reduce-motion * {