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/game/helpers_test.go

@@ -55,16 +55,16 @@ func TestPayloadExchangeRoundTrip(t *testing.T) {
 }
 
 func TestHintsRemaining(t *testing.T) {
-	cases := []struct{ allowance, used, wallet, want int }{
-		{1, 0, 3, 4},
-		{1, 1, 3, 3},
-		{1, 2, 3, 3}, // used past allowance clamps to 0
-		{0, 0, 5, 5},
-		{2, 1, 0, 1},
+	cases := []struct{ allowance, used, want int }{
+		{1, 0, 1},
+		{1, 1, 0},
+		{1, 2, 0}, // used past allowance clamps to 0
+		{3, 1, 2},
+		{0, 0, 0},
 	}
 	for _, c := range cases {
-		if got := hintsRemaining(c.allowance, c.used, c.wallet); got != c.want {
-			t.Errorf("hintsRemaining(%d,%d,%d) = %d, want %d", c.allowance, c.used, c.wallet, got, c.want)
+		if got := hintsRemaining(c.allowance, c.used); got != c.want {
+			t.Errorf("hintsRemaining(%d,%d) = %d, want %d", c.allowance, c.used, got, c.want)
 		}
 	}
 }