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/internal/postgres/migrations/00012_robot_friend_requests.sql

@@ -0,0 +1,28 @@
+-- +goose Up
+-- Per-game friend requests sent to a disguised-robot opponent. A disguised robot is a
+-- shared pool account reused across games under different per-game names, so such a
+-- request must never go into `friendships` (that would befriend/await the shared robot
+-- account, leak that it is a bot across the requester's other games under other names,
+-- and pin the in-game "request sent" state to the shared account instead of this seat).
+-- Each request is recorded here against the specific game + seat, snapshotting the name
+-- the player saw, so the in-game scoreboard can re-mark that one seat as already
+-- requested. The robot ignores it (it never becomes a friendship); a background reaper
+-- deletes a row once its game has been finished for more than the retention window.
+SET search_path = backend, pg_catalog;
+
+CREATE TABLE robot_friend_requests (
+    id           uuid        PRIMARY KEY,
+    requester_id uuid        NOT NULL REFERENCES accounts (account_id) ON DELETE CASCADE,
+    game_id      uuid        NOT NULL REFERENCES games (game_id) ON DELETE CASCADE,
+    seat         smallint    NOT NULL,
+    robot_id     uuid        NOT NULL REFERENCES accounts (account_id) ON DELETE CASCADE,
+    display_name text        NOT NULL,
+    created_at   timestamptz NOT NULL DEFAULT now(),
+    UNIQUE (requester_id, game_id, seat)
+);
+
+CREATE INDEX robot_friend_requests_requester_idx ON robot_friend_requests (requester_id);
+
+-- +goose Down
+SET search_path = backend, pg_catalog;
+DROP TABLE robot_friend_requests;