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/inttest/account_test.go

@@ -89,7 +89,8 @@ func TestGetStatsZeroForFreshAccount(t *testing.T) {
 	if err != nil {
 		t.Fatalf("get stats: %v", err)
 	}
-	if (st != account.Stats{}) {
+	zero := st.Wins == 0 && st.Losses == 0 && st.Draws == 0 && st.MaxGamePoints == 0 && st.MaxWordPoints == 0
+	if !zero || len(st.BestMoves) != 0 {
 		t.Fatalf("fresh stats = %+v, want zero", st)
 	}
 }