feat(ui): name the previous player in the your-turn toast

The in-app "your turn" toast (shown when it becomes your turn in another
game) now reads "<Opponent>: Your turn!", naming the player who moved just
before this one — the previous seat in turn order, so it reads the same in
games with any number of players. Falls back to the bare "Your turn" label
when no name is present (an older peer, or a gap event without one).

YourTurnEvent already carries opponent_name end to end: the backend sets it
(game.displayName of the last mover) and the gateway forwards the payload
opaquely, so this is a client-only change — decode the field, thread it
through PushEvent, and pick the localized string.

ui/src/lib/codec.test.ts

@@ -300,6 +300,39 @@ describe('codec', () => {
     expect(inv.variant).toBe('scrabble_en');
   });
 
+  it('decodes a your_turn event, carrying the previous player name for the toast', () => {
+    const b = new Builder(64);
+    const gid = b.createString('g-1');
+    const name = b.createString('Ann');
+    fb.YourTurnEvent.startYourTurnEvent(b);
+    fb.YourTurnEvent.addGameId(b, gid);
+    fb.YourTurnEvent.addOpponentName(b, name);
+    fb.YourTurnEvent.addMoveCount(b, 7);
+    b.finish(fb.YourTurnEvent.endYourTurnEvent(b));
+    expect(decodeEvent('your_turn', b.asUint8Array())).toEqual({
+      kind: 'your_turn',
+      gameId: 'g-1',
+      deadlineUnix: 0,
+      opponentName: 'Ann',
+      moveCount: 7,
+    });
+  });
+
+  it('decodes a your_turn event with no opponent name as an empty string', () => {
+    const b = new Builder(64);
+    const gid = b.createString('g-2');
+    fb.YourTurnEvent.startYourTurnEvent(b);
+    fb.YourTurnEvent.addGameId(b, gid);
+    b.finish(fb.YourTurnEvent.endYourTurnEvent(b));
+    expect(decodeEvent('your_turn', b.asUint8Array())).toEqual({
+      kind: 'your_turn',
+      gameId: 'g-2',
+      deadlineUnix: 0,
+      opponentName: '',
+      moveCount: 0,
+    });
+  });
+
   it('decodes an opponent_joined event (reusing the match_found payload layout)', () => {
     const b = new Builder(64);
     const gid = b.createString('g-open');