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.

gateway/internal/transcode/encode.go

@@ -223,6 +223,7 @@ func toWireState(s backendclient.StateResp) wire.StateView {
 		Rack:           s.Rack,
 		BagLen:         s.BagLen,
 		HintsRemaining: s.HintsRemaining,
+		WalletBalance:  s.WalletBalance,
 		Alphabet:       alphabet,
 	}
 }
@@ -278,6 +279,7 @@ func encodeHintResult(r backendclient.HintResultResp) []byte {
 	fb.HintResultStart(b)
 	fb.HintResultAddMove(b, move)
 	fb.HintResultAddHintsRemaining(b, int32(r.HintsRemaining))
+	fb.HintResultAddWalletBalance(b, int32(r.WalletBalance))
 	b.Finish(fb.HintResultEnd(b))
 	return b.FinishedBytes()
 }