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/game/stats_test.go

@@ -0,0 +1,50 @@
+package game
+
+import (
+	"reflect"
+	"testing"
+
+	"scrabble/backend/internal/account"
+	"scrabble/backend/internal/engine"
+)
+
+// TestMainWordTilesHorizontalWithBlank checks that a horizontal main word is decoded into
+// tiles along its row, that the per-letter value is looked up from the value table, and that
+// a blank cell (here the middle 'a') is rendered with a zero value regardless of its letter
+// value — the blank flag is taken from the placed-blank set, not from this move's own tiles.
+func TestMainWordTilesHorizontalWithBlank(t *testing.T) {
+	rec := engine.MoveRecord{Dir: engine.Horizontal, MainRow: 7, MainCol: 5, Words: []string{"cat"}}
+	blanks := map[[2]int]bool{{7, 6}: true}
+	values := map[string]int{"c": 3, "a": 1, "t": 1}
+	got := mainWordTiles(rec, blanks, values)
+	want := []account.BestMoveTile{
+		{Letter: "c", Value: 3, Blank: false},
+		{Letter: "a", Value: 0, Blank: true},
+		{Letter: "t", Value: 1, Blank: false},
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("mainWordTiles = %+v, want %+v", got, want)
+	}
+}
+
+// TestMainWordTilesVerticalCyrillic checks vertical walk and multi-byte (Cyrillic) letters:
+// the word must be split by rune, not byte, and laid down its column.
+func TestMainWordTilesVerticalCyrillic(t *testing.T) {
+	rec := engine.MoveRecord{Dir: engine.Vertical, MainRow: 3, MainCol: 8, Words: []string{"съёмка"}}
+	values := map[string]int{"с": 1, "ъ": 10, "ё": 4, "м": 2, "к": 2, "а": 1}
+	got := mainWordTiles(rec, map[[2]int]bool{}, values)
+	want := []account.BestMoveTile{
+		{Letter: "с", Value: 1}, {Letter: "ъ", Value: 10}, {Letter: "ё", Value: 4},
+		{Letter: "м", Value: 2}, {Letter: "к", Value: 2}, {Letter: "а", Value: 1},
+	}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("mainWordTiles = %+v, want %+v", got, want)
+	}
+}
+
+// TestMainWordTilesNoWord checks the defensive nil for a record carrying no words.
+func TestMainWordTilesNoWord(t *testing.T) {
+	if got := mainWordTiles(engine.MoveRecord{}, nil, nil); got != nil {
+		t.Errorf("mainWordTiles(no words) = %+v, want nil", got)
+	}
+}