UI: fix last-move highlight, localize move history, clamp zoom overscroll

- Highlight tracks the last move overall (not the last word): a trailing
  pass/exchange now highlights nothing, so the board no longer lights up the
  opponent's old word after our own empty move.
- Make the highlight event-driven: refreshed only on a real game event
  (open/refresh, opponent move, our own committed move) and dismissed the moment
  composing starts, so recalling a just-placed tile never re-triggers it.
- Localize non-play move-history labels via new move.* catalog keys
  (pass/exchange/resign/timeout); the label printed the raw English action.
- Clamp the zoomed board's pan at its edge (overscroll-behavior: none), removing
  the native rubber-band past the content.

Tests: lastMoveCells unit coverage (trailing pass/exchange -> empty), i18n RU
label assertions, an e2e overscroll-contract check on the zoomed viewport.

ui/src/lib/i18n.test.ts

@@ -13,6 +13,12 @@ describe('i18n catalog', () => {
     expect(translate('ru', 'game.bag', { n: 7 })).toBe('7 в мешке');
   });
 
+  it('localizes move-history action labels', () => {
+    expect(translate('ru', 'move.exchange')).toBe('Обмен');
+    expect(translate('ru', 'move.pass')).toBe('Пас');
+    expect(translate('en', 'move.exchange')).toBe('Exchange');
+  });
+
   it('maps error codes to keys with a generic fallback', () => {
     expect(errorKey('not_your_turn')).toBe('error.not_your_turn');
     expect(errorKey('totally_unknown')).toBe('error.generic');