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/game/Board.svelte

@@ -228,6 +228,9 @@
   }
   .viewport.zoomed {
     overflow: auto;
+    /* Clamp the pan at the board edge: kill the native rubber-band/overscroll so the zoomed
+       board cannot be dragged past its content into empty space — it just stops at the edge. */
+    overscroll-behavior: none;
   }
   /* The query container is the (zoom-scaled) board, so cqw labels scale WITH the board
      — a magnifying-glass zoom. */