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.

gateway/internal/backendclient/api.go

@@ -197,7 +197,6 @@ type StateResp struct {
 	Rack                  []int               `json:"rack"`
 	BagLen                int                 `json:"bag_len"`
 	HintsRemaining        int                 `json:"hints_remaining"`
-	WalletBalance         int                 `json:"wallet_balance"`
 	HintUnlockLeftSeconds int                 `json:"hint_unlock_left_seconds"`
 	Alphabet              []AlphabetEntryJSON `json:"alphabet,omitempty"`
 }