Stage 17 round 5 — backend/correctness bug fixes

- Resign on the opponent's turn: engine ResignSeat(seat) resigns a specific seat
  (not just toMove); game.Resign bypasses the turn check and forfeits the actor's seat.
- Quick-match cancel was a UI no-op (only stopped polling): add the full path
  (REST /lobby/cancel -> gateway lobby.cancel -> client) and clear the matchmaker's
  pending result on Cancel, so a cancelled search is dequeued (no 'already queued', no
  later robot-substituted game). NewGame dequeues on cancel and on abandon.
- Lobby win/loss: result.ts ranked by score, so a 0-0 resignation read as a win.
  The winner now takes rank 1 and the viewer is placed from rank 2 — matching the
  game-detail screen.
- Friend request to a robot: robots no longer block requests; the request stays
  pending and expires (friendRequestTTL), mirroring a human who ignores it.
- Nudge cooldown: ErrNudgeTooSoon now maps to a distinct nudge_too_soon code with a
  correct message; the chat nudge button disables during the hourly cooldown; the
  nudge note reads 'Waiting for your move!' (button keeps the Nudge action label).
Tests: engine/service off-turn resign, matchmaker cancel-clears-result, friend-to-robot
inttest, result.ts 0-0 resignation, nudge_too_soon mapping.

backend/internal/lobby/matchmaker.go

@@ -142,11 +142,14 @@ func (m *Matchmaker) Poll(_ context.Context, accountID uuid.UUID) (EnqueueResult
 	return EnqueueResult{}, nil
 }
 
-// Cancel removes accountID from whatever pool it waits in, reporting whether it
-// was queued.
+// Cancel removes accountID from whatever pool it waits in and drops any pending
+// matched result, reporting whether it was queued. Clearing the result closes the
+// race where the reaper substituted a robot just before the player cancelled: the
+// stale game must not later surface through Poll as a game the player did not want.
 func (m *Matchmaker) Cancel(_ context.Context, accountID uuid.UUID) bool {
 	m.mu.Lock()
 	defer m.mu.Unlock()
+	delete(m.results, accountID)
 	variant, ok := m.queued[accountID]
 	if !ok {
 		return false