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/transcode/transcode_social_test.go

@@ -194,7 +194,11 @@ func TestStatsRoundTrip(t *testing.T) {
 		if r.URL.Path != "/api/v1/user/stats" {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
-		_, _ = w.Write([]byte(`{"wins":5,"losses":3,"draws":1,"max_game_points":420,"max_word_points":90}`))
+		_, _ = w.Write([]byte(`{"wins":5,"losses":3,"draws":1,"max_game_points":420,"max_word_points":90,` +
+			`"best_moves":[{"variant":"scrabble_en","score":90,"word":[` +
+			`{"letter":"c","value":3,"blank":false},` +
+			`{"letter":"a","value":0,"blank":true},` +
+			`{"letter":"t","value":1,"blank":false}]}]}`))
 	})
 	defer cleanup()
 
@@ -208,6 +212,20 @@ func TestStatsRoundTrip(t *testing.T) {
 	if st.Wins() != 5 || st.Losses() != 3 || st.Draws() != 1 || st.MaxGamePoints() != 420 || st.MaxWordPoints() != 90 {
 		t.Fatalf("stats decoded wrong: %+v", st)
 	}
+	if st.BestMovesLength() != 1 {
+		t.Fatalf("best moves length = %d, want 1", st.BestMovesLength())
+	}
+	var bm fb.BestMoveView
+	st.BestMoves(&bm, 0)
+	if string(bm.Variant()) != "scrabble_en" || bm.Score() != 90 || bm.WordLength() != 3 {
+		t.Fatalf("best move decoded wrong: variant=%q score=%d wordLen=%d", bm.Variant(), bm.Score(), bm.WordLength())
+	}
+	// The middle tile is a blank: it carries its designated letter but scores 0.
+	var tile fb.BestMoveTile
+	bm.Word(&tile, 1)
+	if string(tile.Letter()) != "a" || tile.Value() != 0 || !tile.Blank() {
+		t.Fatalf("blank tile decoded wrong: letter=%q value=%d blank=%v", tile.Letter(), tile.Value(), tile.Blank())
+	}
 }
 
 func TestGcgRoundTrip(t *testing.T) {