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.

gateway/internal/backendclient/api_social.go

@@ -47,13 +47,31 @@ type BlockListResp struct {
 	Blocked []AccountRefResp `json:"blocked"`
 }
 
-// StatsResp is a durable account's lifetime statistics.
+// StatsResp is a durable account's lifetime statistics. BestMoves breaks the best move
+// down per variant, carrying the word itself; it is absent for an account with no recorded
+// play and lists only variants the account has played.
 type StatsResp struct {
-	Wins          int `json:"wins"`
-	Losses        int `json:"losses"`
-	Draws         int `json:"draws"`
-	MaxGamePoints int `json:"max_game_points"`
-	MaxWordPoints int `json:"max_word_points"`
+	Wins          int            `json:"wins"`
+	Losses        int            `json:"losses"`
+	Draws         int            `json:"draws"`
+	MaxGamePoints int            `json:"max_game_points"`
+	MaxWordPoints int            `json:"max_word_points"`
+	BestMoves     []BestMoveResp `json:"best_moves"`
+}
+
+// BestMoveResp is one variant's best play: the variant label, the play's total score and
+// its main word as ordered tiles (letter, value, blank — value 0 for a blank).
+type BestMoveResp struct {
+	Variant string             `json:"variant"`
+	Score   int                `json:"score"`
+	Word    []BestMoveTileResp `json:"word"`
+}
+
+// BestMoveTileResp is one letter cell of a best-move word.
+type BestMoveTileResp struct {
+	Letter string `json:"letter"`
+	Value  int    `json:"value"`
+	Blank  bool   `json:"blank"`
 }
 
 // InvitationInviteeResp is one invitee's seat and response with their name.