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/README.md

@@ -45,6 +45,9 @@ including obfuscated forms) and stored with the sender's IP. Each message carrie
 `unread_seats` read bitmask (a set bit per recipient seat still to read it); `MarkRead`
 clears a reader's bit when they open the move history or chat, and a wired `NudgeClearer`
 clears a nudge when its recipient moves — both record the publish-to-read latency.
+A friend request (or block) aimed at a **disguised pooled robot** is recorded per game+seat
+in `robot_friend_requests` / `robot_blocks`, never against the shared robot account; a
+background reaper drops a robot friend request once its game has been finished for **7 days**.
 `internal/account`
 gains profile editing and the email confirm-code flow (a `Mailer` seam: SMTP or a
 development log mailer). The engine now also handles **multi-player drop-out**: in