feat(stats): show the best move word per game variant

Replace the single "best move" number on the statistics screen with a
full-width per-variant breakdown: the highest-scoring play in each variant
the player has played, drawn as game tiles (a wildcard shows its letter but
no value), with the words and scores right-aligned to shared edges.

- backend: new account_best_move table (PK account_id+variant) keeping the
  main word as JSON tiles {letter,value,blank}; captured at game finish in
  buildStats (blank flags taken from every placed blank — equivalent to the
  final board), upserted in the finish transaction and replaced only by a
  strictly higher-scoring play. Guest/honest-AI games still record nothing.
  GetStats + statsDTO expose best_moves.
- wire: StatsView gains best_moves:[BestMoveView{variant,score,word:[BestMoveTile]}]
  (trailing, backward-compatible); gateway encodeStats + UI codec updated.
- ui: new WordTiles component (board's tile look, fixed px size); Stats.svelte
  drops the maxWord card and adds the full-width best-move card (catalogue
  order, empty variants omitted).
- docs: ARCHITECTURE §9 + schema, FUNCTIONAL (+ru), UI_DESIGN.

Tests: mainWordTiles unit + buildStats end-to-end (inttest) + gateway and UI
codec round-trips (incl. a blank tile) + e2e.

pkg/fbs/scrabble.fbs

@@ -458,14 +458,35 @@ table LinkResult {
   session:Session;
 }
 
+// BestMoveTile is one letter cell of a best-move word: its concrete letter (the designated
+// letter for a blank), its tile point value (0 for a blank) and whether it is a blank. It
+// lets the client render the word as game tiles without the variant's alphabet table.
+table BestMoveTile {
+  letter:string;
+  value:int;
+  blank:bool;
+}
+
+// BestMoveView is an account's highest-scoring single play within one game variant: the
+// variant label, the play's total score (matching max_word_points for that variant) and its
+// main word as ordered tiles.
+table BestMoveView {
+  variant:string;
+  score:int;
+  word:[BestMoveTile];
+}
+
 // StatsView is a durable account's lifetime statistics (games-played and win-rate
-// are derived client-side).
+// are derived client-side). best_moves breaks the best move down per variant, carrying the
+// word itself; it is empty for an account with no recorded play and lists only variants the
+// account has played (added trailing — backward-compatible).
 table StatsView {
   wins:int;
   losses:int;
   draws:int;
   max_game_points:int;
   max_word_points:int;
+  best_moves:[BestMoveView];
 }
 
 // TargetRequest names a single counterpart account (friend request/cancel/unfriend,