Stage 7: regression tests for the polished UI (logic + behaviour)

Lock the polish branch's behaviour so a future UI edit surfaces as a failing
assertion to re-agree or fix.

Unit (vitest, node env):
- placement: recallIndex, cellOccupied/isBlankSlot, non-linear direction, the
  single-tile submit default, and placementFromHint blank-fallback / rack-exhausted.
- banner: the marquee scroll-cycle repeat-then-advance, stop(), root-relative and
  multiple links.
- client.GatewayError. Extract the check-word constraints out of Game.svelte into a
  pure lib/checkword.ts (sanitize + canCheck) and cover them.

E2E (playwright mock, Chromium + WebKit):
- commit via the 🏁 control, history slide-down + close, the exchange dialog,
  check-word input sanitising + verdict, resign-to-finished, and the Settings
  board-label mode changing the on-board labels.

ui/src/game/Game.svelte

@@ -15,6 +15,7 @@
   import type { ChatMessage, Direction, EvalResult, MoveRecord, StateView } from '../lib/model';
   import { replay } from '../lib/board';
   import { alphabet, centre, premiumGrid } from '../lib/premiums';
+  import { canCheckWord, sanitizeCheckWord } from '../lib/checkword';
   import {
     BLANK,
     newPlacement,
@@ -311,15 +312,12 @@
     checkOpen = true;
   }
   function onCheckInput(e: Event) {
-    const allowed = new Set(alphabet(variant));
-    const raw = (e.target as HTMLInputElement).value.toUpperCase();
-    checkWord = Array.from(raw).filter((ch) => allowed.has(ch)).slice(0, 15).join('');
+    checkWord = sanitizeCheckWord((e.target as HTMLInputElement).value, alphabet(variant));
   }
   // Check is disabled while cooling down, for an already-checked word, or an out-of-range
   // length. The input filter already restricts to the variant's alphabet.
   function canCheck(): boolean {
-    const w = checkWord.trim();
-    return w.length >= 2 && w.length <= 15 && !checkedWords.has(w.toUpperCase()) && !cooling;
+    return canCheckWord(checkWord, checkedWords.has(checkWord.trim().toUpperCase()), cooling);
   }
   async function runCheck() {
     if (!canCheck()) return;