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.

ui/src/lib/model.ts

@@ -206,13 +206,33 @@ export interface FriendCode {
   expiresAtUnix: number;
 }
 
-/** A durable account's lifetime statistics. */
+/** One letter cell of a best-move word: its display letter, its tile value (0 for a
+ *  blank) and whether it is a blank — enough to render it as a game tile without the
+ *  variant's alphabet table. */
+export interface BestMoveTile {
+  letter: string;
+  value: number;
+  blank: boolean;
+}
+
+/** An account's highest-scoring single play within one variant: the variant, the play's
+ *  total score and its main word as ordered tiles. */
+export interface BestMove {
+  variant: Variant;
+  score: number;
+  word: BestMoveTile[];
+}
+
+/** A durable account's lifetime statistics. bestMoves breaks the best move down per
+ *  variant (with the word itself); it is empty for an account with no recorded play and
+ *  lists only variants the account has played. */
 export interface Stats {
   wins: number;
   losses: number;
   draws: number;
   maxGamePoints: number;
   maxWordPoints: number;
+  bestMoves: BestMove[];
 }
 
 /** Settings the inviter chooses for a friend game. */