feat(hint): unify the vs_ai idle hint online + offline (server-enforced, monotonic)

Online vs_ai hints were broken: #207 made the vs_ai hint button always-enabled and
wallet-free (assuming all vs_ai = the offline idle-gate), but the backend still served
online vs_ai from the allowance/wallet, so over-clicking hit ErrNoHintsLeft -> a generic
error toast. Now online vs_ai uses the SAME idle-gate model as offline.

Backend: Hint() for a vs_ai game skips the allowance/wallet and increments no hints_used
(owner: vs_ai counts toward no hint statistic), and is idle-gated from the SERVER clock --
it returns ErrHintLocked (code hint_locked) until the robot's last move + 30 min, else
serves the top move. GameState/StateView expose hint_unlock_left_seconds (server-computed
seconds remaining; 0 for a human game / first move / not-your-turn). Pure helper
hintUnlockLeftSeconds unit-tested.

Wire: StateView gains hint_unlock_left_seconds (FlatBuffers, additive); pkg/wire + gateway
transcode carry it (round-trip test).

Client: the gate counts down from a MONOTONIC clock (performance.now()) anchored to the
source's seconds-left when it lands (on load from the view; to the full window when the
robot moves), so a client clock change cannot skew it and a relaunch re-reads a fresh
value. The vs_ai hint button (online + offline) shows the lock + toast; doHint handles the
hint_locked backstop by re-syncing. Replaces #207's absolute hintUnlockAtMs on the
wire/model/delta (the offline record keeps the absolute for persistence; the view exposes
seconds-left).

Docs FUNCTIONAL(+_ru)/ARCHITECTURE updated. Verified: go build/vet + game/server/transcode
tests (+ new hintUnlockLeftSeconds); ui check 0 / unit 490 / e2e 198 (one pre-existing
webkit offline flake) / app entry 114.2/115.

gateway/internal/transcode/transcode_test.go

@@ -59,7 +59,7 @@ func TestGameStateRoundTripForwardsUserID(t *testing.T) {
 		if r.URL.Path != "/api/v1/user/games/g-1/state" {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
-		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"scrabble_en","status":"active","players":2,"to_move":1,"seats":[{"seat":0,"account_id":"u-7","score":5}]},"seat":0,"rack":[0,1],"bag_len":80,"hints_remaining":4,"wallet_balance":3}`))
+		_, _ = w.Write([]byte(`{"game":{"id":"g-1","variant":"scrabble_en","status":"active","players":2,"to_move":1,"seats":[{"seat":0,"account_id":"u-7","score":5}]},"seat":0,"rack":[0,1],"bag_len":80,"hints_remaining":4,"wallet_balance":3,"hint_unlock_left_seconds":1200}`))
 	})
 	defer cleanup()
 
@@ -77,8 +77,8 @@ func TestGameStateRoundTripForwardsUserID(t *testing.T) {
 		t.Fatalf("handler: %v", err)
 	}
 	st := fb.GetRootAsStateView(payload, 0)
-	if st.BagLen() != 80 || st.RackLength() != 2 || st.HintsRemaining() != 4 || st.WalletBalance() != 3 {
-		t.Fatalf("state decoded wrong: bag=%d rack=%d hints=%d wallet=%d", st.BagLen(), st.RackLength(), st.HintsRemaining(), st.WalletBalance())
+	if st.BagLen() != 80 || st.RackLength() != 2 || st.HintsRemaining() != 4 || st.WalletBalance() != 3 || st.HintUnlockLeftSeconds() != 1200 {
+		t.Fatalf("state decoded wrong: bag=%d rack=%d hints=%d wallet=%d unlockLeft=%d", st.BagLen(), st.RackLength(), st.HintsRemaining(), st.WalletBalance(), st.HintUnlockLeftSeconds())
 	}
 	game := st.Game(nil)
 	if game == nil || string(game.Id()) != "g-1" || string(game.Variant()) != "scrabble_en" || game.ToMove() != 1 {