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.

gateway/internal/transcode/transcode_social_test.go

@@ -59,7 +59,8 @@ func TestFriendsOutgoingRoundTrip(t *testing.T) {
 		if r.URL.Path != "/api/v1/user/friends/outgoing" {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
-		_, _ = w.Write([]byte(`{"requests":[{"account_id":"o-1","display_name":"Pat"}]}`))
+		_, _ = w.Write([]byte(`{"requests":[{"account_id":"o-1","display_name":"Pat"}],` +
+			`"robots":[{"id":"r-1","display_name":"Robbie","game_id":"g-1","seat":1}]}`))
 	})
 	defer cleanup()
 
@@ -81,6 +82,15 @@ func TestFriendsOutgoingRoundTrip(t *testing.T) {
 	if string(ref.AccountId()) != "o-1" || string(ref.DisplayName()) != "Pat" {
 		t.Fatalf("outgoing[0] = (%q, %q), want (o-1, Pat)", ref.AccountId(), ref.DisplayName())
 	}
+	// The per-game disguised-robot requests round-trip in their own vector.
+	if ol.RobotsLength() != 1 {
+		t.Fatalf("robots length = %d, want 1", ol.RobotsLength())
+	}
+	var rr fb.RobotFriendRef
+	ol.Robots(&rr, 0)
+	if string(rr.Id()) != "r-1" || string(rr.DisplayName()) != "Robbie" || string(rr.GameId()) != "g-1" || rr.Seat() != 1 {
+		t.Fatalf("robot[0] = (%q, %q, %q, %d), want (r-1, Robbie, g-1, 1)", rr.Id(), rr.DisplayName(), rr.GameId(), rr.Seat())
+	}
 }
 
 func TestFriendRequestForwardsTarget(t *testing.T) {