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.

pkg/fbs/scrabblefb/StateView.go

@@ -156,8 +156,20 @@ func (rcv *StateView) MutateWalletBalance(n int32) bool {
 	return rcv._tab.MutateInt32Slot(16, n)
 }
 
+func (rcv *StateView) HintUnlockLeftSeconds() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(18))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *StateView) MutateHintUnlockLeftSeconds(n int32) bool {
+	return rcv._tab.MutateInt32Slot(18, n)
+}
+
 func StateViewStart(builder *flatbuffers.Builder) {
-	builder.StartObject(7)
+	builder.StartObject(8)
 }
 func StateViewAddGame(builder *flatbuffers.Builder, game flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(game), 0)
@@ -186,6 +198,9 @@ func StateViewStartAlphabetVector(builder *flatbuffers.Builder, numElems int) fl
 func StateViewAddWalletBalance(builder *flatbuffers.Builder, walletBalance int32) {
 	builder.PrependInt32Slot(6, walletBalance, 0)
 }
+func StateViewAddHintUnlockLeftSeconds(builder *flatbuffers.Builder, hintUnlockLeftSeconds int32) {
+	builder.PrependInt32Slot(7, hintUnlockLeftSeconds, 0)
+}
 func StateViewEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }