feat(stats): best-move word, moves & hint-share, and a hint-count fix (#81)

The statistics screen gains real depth, plus a hint-count bug fix found along the way.

- Best move per variant: the screen shows the actual best-move word (drawn as game
  tiles; a wildcard shows its letter but no value), broken down by game variant, empty
  variants omitted. New account_best_move table, written at game finish.
- Moves & hint share: two new lifetime tiles — the player's play count and the share of
  plays that used a hint — from summed account_stats counters (moves, hints_used).
  Honest-AI games are excluded, like the rest of the stats.
- Hint-count fix: the in-game hint badge no longer goes stale across games. The global
  wallet now rides the wire apart from the per-game allowance (wallet_balance on
  StateView/HintResult/StatsView), so the client reads the live wallet rather than a
  per-game snapshot; game_players.hints_used now counts every hint (allowance + wallet),
  its true per-game total.
- Account merge: sums the new moves/hints_used counters and merges the per-variant best
  moves (higher score kept), which it previously dropped.
- Admin: the user card shows Moves and Hints used.
- UI polish: tab/label wording, game-over text, and e2e selectors hardened against label
  changes.

All wire additions are trailing (backward-compatible). Docs (ARCHITECTURE, FUNCTIONAL +ru,
UISN_DESIGN) updated in step.

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)
+	}
+}