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.

gateway/internal/transcode/transcode.go

@@ -24,6 +24,7 @@ const (
 	MsgGameSubmitPlay = "game.submit_play"
 	MsgGameState      = "game.state"
 	MsgLobbyEnqueue   = "lobby.enqueue"
+	MsgLobbyCancel    = "lobby.cancel"
 	MsgLobbyPoll      = "lobby.poll"
 	MsgChatPost       = "chat.post"
 	MsgGamesList      = "games.list"
@@ -93,6 +94,7 @@ func NewRegistry(backend *backendclient.Client, tg TelegramValidator, defaultLan
 	r.ops[MsgGameSubmitPlay] = Op{Handler: submitPlayHandler(backend), Auth: true}
 	r.ops[MsgGameState] = Op{Handler: gameStateHandler(backend), Auth: true}
 	r.ops[MsgLobbyEnqueue] = Op{Handler: enqueueHandler(backend), Auth: true}
+	r.ops[MsgLobbyCancel] = Op{Handler: cancelHandler(backend), Auth: true}
 	r.ops[MsgLobbyPoll] = Op{Handler: pollHandler(backend), Auth: true}
 	r.ops[MsgChatPost] = Op{Handler: chatPostHandler(backend), Auth: true}
 	r.ops[MsgGamesList] = Op{Handler: gamesListHandler(backend), Auth: true}
@@ -233,6 +235,17 @@ func pollHandler(backend *backendclient.Client) Handler {
 	}
 }
 
+// cancelHandler removes the caller from the auto-match pool. It carries no result;
+// it echoes an empty (unmatched) Match so the client has a well-formed payload.
+func cancelHandler(backend *backendclient.Client) Handler {
+	return func(ctx context.Context, req Request) ([]byte, error) {
+		if err := backend.Cancel(ctx, req.UserID); err != nil {
+			return nil, err
+		}
+		return encodeMatch(backendclient.MatchResp{}), nil
+	}
+}
+
 func chatPostHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsChatPostRequest(req.Payload, 0)