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.

backend/internal/server/handlers_account.go

@@ -29,13 +29,25 @@ type updateProfileRequest struct {
 }
 
 // statsDTO is a durable account's lifetime statistics (the derived games-played and
-// win-rate are computed client-side).
+// win-rate are computed client-side). BestMoves breaks the best move down per variant,
+// carrying the word itself; it is absent for an account with no recorded play and never
+// lists a variant the account has not played.
 type statsDTO 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     []bestMoveDTO `json:"best_moves,omitempty"`
+}
+
+// bestMoveDTO 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), so the
+// client renders it as game tiles without the variant's alphabet table.
+type bestMoveDTO struct {
+	Variant string                 `json:"variant"`
+	Score   int                    `json:"score"`
+	Word    []account.BestMoveTile `json:"word"`
 }
 
 // parseAwayTime parses an "HH:MM" away-window bound.
@@ -140,11 +152,19 @@ func (s *Server) handleStats(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
+	var best []bestMoveDTO
+	if len(st.BestMoves) > 0 {
+		best = make([]bestMoveDTO, len(st.BestMoves))
+		for i, b := range st.BestMoves {
+			best[i] = bestMoveDTO{Variant: b.Variant, Score: b.Score, Word: b.Tiles}
+		}
+	}
 	c.JSON(http.StatusOK, statsDTO{
 		Wins:          st.Wins,
 		Losses:        st.Losses,
 		Draws:         st.Draws,
 		MaxGamePoints: st.MaxGamePoints,
 		MaxWordPoints: st.MaxWordPoints,
+		BestMoves:     best,
 	})
 }