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/server/handlers.go

@@ -69,6 +69,7 @@ func (s *Server) registerRoutes() {
 	}
 	if s.matchmaker != nil {
 		u.POST("/lobby/enqueue", s.handleEnqueue)
+		u.POST("/lobby/cancel", s.handleCancel)
 		u.GET("/lobby/poll", s.handlePoll)
 	}
 	if s.invitations != nil {
@@ -200,9 +201,12 @@ func statusForError(err error) (int, string) {
 	case errors.Is(err, session.ErrNotFound):
 		return http.StatusUnauthorized, "session_invalid"
 	case errors.Is(err, social.ErrChatBlocked), errors.Is(err, social.ErrMessageTooLong),
-		errors.Is(err, social.ErrEmptyMessage), errors.Is(err, social.ErrForbiddenContent),
-		errors.Is(err, social.ErrNudgeTooSoon):
+		errors.Is(err, social.ErrEmptyMessage), errors.Is(err, social.ErrForbiddenContent):
 		return http.StatusUnprocessableEntity, "chat_rejected"
+	case errors.Is(err, social.ErrNudgeTooSoon):
+		// A too-frequent nudge is a distinct, non-content rejection — the UI must say
+		// "don't rush the player so often", not the chat content-rejection message.
+		return http.StatusConflict, "nudge_too_soon"
 	case errors.Is(err, social.ErrSelfRelation):
 		return http.StatusBadRequest, "self_relation"
 	case errors.Is(err, social.ErrRequestExists):