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/service.go

@@ -1419,19 +1419,41 @@ func replayMove(g *engine.Game, mv HistoryMove) error {
 }
 
 // buildStats derives each seat's statistics contribution from a finished game:
-// win/loss/draw from the (resignation-aware) winner, the final score, and the
-// best single-move score from the log.
+// win/loss/draw from the (resignation-aware) winner, the final score, and the best
+// single play from the log — its score and, for the per-variant breakdown, its main
+// word as rendering tiles. Blank flags are taken from every blank ever placed (so a
+// blank laid by an earlier move and embedded in the best word is honoured), which is
+// equivalent to reading the final board since a placed tile never moves.
 func buildStats(g *engine.Game, seats []Seat) []statDelta {
 	res := g.Result()
-	best := make(map[int]int)
+	bestRec := make(map[int]engine.MoveRecord)
+	blanks := make(map[[2]int]bool)
 	for _, rec := range g.Log() {
-		if rec.Action == engine.ActionPlay && rec.Score > best[rec.Player] {
-			best[rec.Player] = rec.Score
+		if rec.Action != engine.ActionPlay {
+			continue
+		}
+		for _, t := range rec.Tiles {
+			if t.Blank {
+				blanks[[2]int{t.Row, t.Col}] = true
+			}
+		}
+		if cur, ok := bestRec[rec.Player]; !ok || rec.Score > cur.Score {
+			bestRec[rec.Player] = rec
 		}
 	}
+	variant := g.Variant().String()
+	values := letterValues(g.Variant())
 	out := make([]statDelta, 0, len(seats))
 	for _, s := range seats {
-		d := statDelta{accountID: s.AccountID, gamePoints: g.Score(s.Seat), wordPoints: best[s.Seat]}
+		d := statDelta{accountID: s.AccountID, gamePoints: g.Score(s.Seat)}
+		if rec, ok := bestRec[s.Seat]; ok {
+			d.wordPoints = rec.Score
+			if rec.Score > 0 {
+				d.bestVariant = variant
+				d.bestScore = rec.Score
+				d.bestTiles = mainWordTiles(rec, blanks, values)
+			}
+		}
 		switch {
 		case res.Winner < 0:
 			d.draws = 1
@@ -1445,6 +1467,49 @@ func buildStats(g *engine.Game, seats []Seat) []statDelta {
 	return out
 }
 
+// letterValues builds a lower-cased letter -> tile value lookup for a variant from the
+// engine's alphabet table, so a best-move word can be rendered with per-tile values on a
+// screen (statistics) that has not cached the variant's alphabet. It is empty for an
+// unrecognised variant, leaving every value zero.
+func letterValues(v engine.Variant) map[string]int {
+	table, err := engine.AlphabetTable(v)
+	if err != nil {
+		return nil
+	}
+	m := make(map[string]int, len(table))
+	for _, e := range table {
+		m[strings.ToLower(e.Letter)] = e.Value
+	}
+	return m
+}
+
+// mainWordTiles decodes a play's main word into rendering tiles: each letter with its
+// tile value (0 for a blank) and blank flag. blanks is the set of board coordinates a
+// blank was ever placed on; values maps a lower-cased letter to its tile value. It walks
+// the word from its first-letter coordinate along the play's orientation.
+func mainWordTiles(rec engine.MoveRecord, blanks map[[2]int]bool, values map[string]int) []account.BestMoveTile {
+	if len(rec.Words) == 0 {
+		return nil
+	}
+	dr, dc := 0, 1
+	if rec.Dir == engine.Vertical {
+		dr, dc = 1, 0
+	}
+	letters := []rune(rec.Words[0])
+	out := make([]account.BestMoveTile, len(letters))
+	for i, r := range letters {
+		row, col := rec.MainRow+i*dr, rec.MainCol+i*dc
+		blank := blanks[[2]int{row, col}]
+		letter := string(r)
+		value := 0
+		if !blank {
+			value = values[strings.ToLower(letter)]
+		}
+		out[i] = account.BestMoveTile{Letter: letter, Value: value, Blank: blank}
+	}
+	return out
+}
+
 // nonGuestSeats filters out guest seats so the finish-time statistics are
 // recomputed for durable non-guest accounts only — guests never accrue
 // statistics (docs/ARCHITECTURE.md §9). It is called once per game, on finish.

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)
+	}
+}

backend/internal/game/store.go

@@ -3,6 +3,7 @@ package game
 import (
 	"context"
 	"database/sql"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"hash/fnv"
@@ -12,6 +13,7 @@ import (
 	"github.com/go-jet/jet/v2/qrm"
 	"github.com/google/uuid"
 
+	"scrabble/backend/internal/account"
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/postgres/jet/backend/model"
 	"scrabble/backend/internal/postgres/jet/backend/table"
@@ -53,13 +55,20 @@ type gameInsert struct {
 }
 
 // statDelta is one account's contribution to its statistics on a game finish.
+// bestVariant/bestScore/bestTiles describe the game's best play for this account when
+// it scored (bestVariant empty otherwise): the variant label, the play's total score and
+// its main word as rendering tiles. They feed the per-variant account_best_move upsert,
+// which keeps only the account's highest-scoring play per variant.
 type statDelta struct {
-	accountID  uuid.UUID
-	wins       int
-	losses     int
-	draws      int
-	gamePoints int
-	wordPoints int
+	accountID   uuid.UUID
+	wins        int
+	losses      int
+	draws       int
+	gamePoints  int
+	wordPoints  int
+	bestVariant string
+	bestScore   int
+	bestTiles   []account.BestMoveTile
 }
 
 // commit is everything a single committed transition persists: the journal row,
@@ -633,6 +642,9 @@ func (s *Store) CommitMove(ctx context.Context, c commit) error {
 				if err := upsertStats(ctx, tx, d, c.now); err != nil {
 					return err
 				}
+				if err := upsertBestMove(ctx, tx, d, c.now); err != nil {
+					return err
+				}
 			}
 		}
 		return nil
@@ -662,6 +674,9 @@ func (s *Store) VoidGame(ctx context.Context, v voidCommit) error {
 			if err := upsertStats(ctx, tx, d, v.now); err != nil {
 				return err
 			}
+			if err := upsertBestMove(ctx, tx, d, v.now); err != nil {
+				return err
+			}
 		}
 		return nil
 	})
@@ -728,6 +743,37 @@ func upsertStats(ctx context.Context, tx *sql.Tx, d statDelta, now time.Time) er
 	return nil
 }
 
+// upsertBestMove records the account's best play for a variant, keeping only the
+// highest-scoring one: a first play inserts, a later one replaces it only when it scored
+// strictly higher (the conditional DO UPDATE makes the upsert atomic under concurrent
+// finishes without a separate lock). It is a no-op when the finish carries no scoring play
+// for the account (a draw with no plays, or an exchange/pass-only game).
+func upsertBestMove(ctx context.Context, tx *sql.Tx, d statDelta, now time.Time) error {
+	if d.bestVariant == "" || len(d.bestTiles) == 0 {
+		return nil
+	}
+	tiles, err := json.Marshal(d.bestTiles)
+	if err != nil {
+		return fmt.Errorf("marshal best move %s/%s: %w", d.accountID, d.bestVariant, err)
+	}
+	stmt := table.AccountBestMove.
+		INSERT(
+			table.AccountBestMove.AccountID, table.AccountBestMove.Variant,
+			table.AccountBestMove.Score, table.AccountBestMove.Tiles, table.AccountBestMove.UpdatedAt,
+		).
+		VALUES(d.accountID, d.bestVariant, d.bestScore, string(tiles), postgres.TimestampzT(now)).
+		ON_CONFLICT(table.AccountBestMove.AccountID, table.AccountBestMove.Variant).
+		DO_UPDATE(postgres.SET(
+			table.AccountBestMove.Score.SET(table.AccountBestMove.EXCLUDED.Score),
+			table.AccountBestMove.Tiles.SET(table.AccountBestMove.EXCLUDED.Tiles),
+			table.AccountBestMove.UpdatedAt.SET(table.AccountBestMove.EXCLUDED.UpdatedAt),
+		).WHERE(table.AccountBestMove.EXCLUDED.Score.GT(table.AccountBestMove.Score)))
+	if _, err := stmt.ExecContext(ctx, tx); err != nil {
+		return fmt.Errorf("upsert best move %s/%s: %w", d.accountID, d.bestVariant, err)
+	}
+	return nil
+}
+
 // SpendHintAllowance increments a seat's per-game hint counter by one.
 func (s *Store) SpendHintAllowance(ctx context.Context, gameID uuid.UUID, seat int) error {
 	stmt := table.GamePlayers.