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.

pkg/fbs/scrabblefb/HintResult.go

@@ -66,8 +66,20 @@ func (rcv *HintResult) MutateHintsRemaining(n int32) bool {
 	return rcv._tab.MutateInt32Slot(6, n)
 }
 
+func (rcv *HintResult) WalletBalance() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *HintResult) MutateWalletBalance(n int32) bool {
+	return rcv._tab.MutateInt32Slot(8, n)
+}
+
 func HintResultStart(builder *flatbuffers.Builder) {
-	builder.StartObject(2)
+	builder.StartObject(3)
 }
 func HintResultAddMove(builder *flatbuffers.Builder, move flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(move), 0)
@@ -75,6 +87,9 @@ func HintResultAddMove(builder *flatbuffers.Builder, move flatbuffers.UOffsetT)
 func HintResultAddHintsRemaining(builder *flatbuffers.Builder, hintsRemaining int32) {
 	builder.PrependInt32Slot(1, hintsRemaining, 0)
 }
+func HintResultAddWalletBalance(builder *flatbuffers.Builder, walletBalance int32) {
+	builder.PrependInt32Slot(2, walletBalance, 0)
+}
 func HintResultEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }