feat(social): per-game friend request to disguised robots + lobby/stats/tile cosmetics

Functional: the in-game add-friend handshake aimed at an auto-match opponent who is
secretly a pooled robot now records the request per (game, seat) in a new
robot_friend_requests table -- never against the shared robot account -- mirroring the
robot_blocks pattern. The shared account stays out of friendships, the "requested" state
is pinned to the seat (not leaked across the player's other games), the robot ignores it,
and a background reaper drops the row 7 days after its game finishes. The outgoing-requests
list carries these per-game rows so the seat control stays disabled across reloads. No
withdraw UI, per owner decision.

Cosmetics:
- Lobby: an in-progress game tints the viewer's own score number green when leading or
  tied, red when trailing (reusing --ok/--danger); scores now render in seat-number order,
  matching the over-the-board scoreboard.
- Stats: the per-variant best move is laid out on two lines -- the variant label, then the
  score (right-aligned in its column) and the word tiles (left-aligned) below it.
- Dark theme: the played-tile background is a touch darker so a player-placed tile reads
  with more contrast (light theme unchanged).

Docs (ARCHITECTURE, FUNCTIONAL + _ru mirror, backend README, UI_DESIGN) updated in the
same change.

backend/cmd/backend/main.go

@@ -171,6 +171,13 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	socialSvc.SetMetrics(tel.MeterProvider().Meter("scrabble/backend/social"))
 	// A nudge the recipient answered by moving is marked read on the move path.
 	games.SetNudgeClearer(socialSvc.ClearNudges)
+	// Reap per-game disguised-robot friend requests once their game is long finished
+	// (the robot ignores them; the row only pins the in-game "request sent" state).
+	robotReqReaper := social.NewRobotFriendRequestReaper(socialSvc, logger)
+	go robotReqReaper.Run(ctx)
+	logger.Info("robot friend request reaper started",
+		zap.Duration("interval", robotReqReaper.Interval()),
+		zap.Duration("retention", robotReqReaper.Retention()))
 	feedbackSvc := feedback.NewService(feedback.NewStore(db), accounts)
 	feedbackSvc.SetNotifier(hub)