fix(game): show granted/bought hints in-game — finish the D31 wire removal

The in-game hint badge ignored the payments hint wallet: loading a game clobbered
app.profile.hintBalance with the deprecated StateView.wallet_balance (zeroed in the
D31 domain removal but left in the protocol as a dead 0, then synced over the real
balance at game load).

Finish the removal honestly. StateView carries the per-game allowance alone
(hints_remaining); the purchasable wallet lives solely on the profile and the client
adds it (lib/hints). Removed StateView.wallet_balance across every layer — backend
StateView/DTO, notify.PlayerState (live events), gateway StateResp + wire.StateView,
the client model/codec/mock/localgame — and dropped the now-unused wallet arg from
hintsRemaining. The FBS field is tombstoned `(deprecated)` (not deleted) so the vtable
slots after it stay stable across a rolling deploy; no accessor is generated. HintResult
keeps wallet_balance (the real post-spend payments balance the client adopts into the
profile). The StateView type no longer has walletBalance, so the clobber cannot return
without a compile error.

Tests: hintsRemaining (2-arg); hints.hintsLeft (allowance + live wallet, no strip); the
game state/hint integration (allowance-only HintsRemaining); gateway transcode; FBS regen.

backend/internal/server/dto.go

@@ -177,7 +177,6 @@ type stateDTO struct {
 	Rack           []int   `json:"rack"`
 	BagLen         int     `json:"bag_len"`
 	HintsRemaining int     `json:"hints_remaining"`
-	WalletBalance  int     `json:"wallet_balance"`
 	// HintUnlockLeftSeconds is the vs_ai idle-hint gate: seconds until the hint unlocks (0 for a human
 	// game / first move / not your turn). The client anchors a monotonic countdown to it.
 	HintUnlockLeftSeconds int                `json:"hint_unlock_left_seconds"`
@@ -334,7 +333,6 @@ func stateDTOFrom(v game.StateView, includeAlphabet bool) (stateDTO, error) {
 		Rack:                  rack,
 		BagLen:                v.BagLen,
 		HintsRemaining:        v.HintsRemaining,
-		WalletBalance:         v.WalletBalance,
 		HintUnlockLeftSeconds: v.HintUnlockLeftSeconds,
 	}
 	if includeAlphabet {