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/wire/build.go

@@ -87,13 +87,14 @@ type AlphabetEntry struct {
 // (wire alphabet indices), bag size and hint budget. Alphabet is set only when the
 // recipient may not have cached the variant's display table yet.
 type StateView struct {
-	Game           GameView
-	Seat           int
-	Rack           []int
-	BagLen         int
-	HintsRemaining int
-	WalletBalance  int
-	Alphabet       []AlphabetEntry
+	Game                  GameView
+	Seat                  int
+	Rack                  []int
+	BagLen                int
+	HintsRemaining        int
+	WalletBalance         int
+	HintUnlockLeftSeconds int
+	Alphabet              []AlphabetEntry
 }
 
 // AccountRef is a referenced account with its display name resolved.
@@ -256,6 +257,7 @@ func BuildStateView(b *flatbuffers.Builder, s StateView) flatbuffers.UOffsetT {
 	fb.StateViewAddBagLen(b, int32(s.BagLen))
 	fb.StateViewAddHintsRemaining(b, int32(s.HintsRemaining))
 	fb.StateViewAddWalletBalance(b, int32(s.WalletBalance))
+	fb.StateViewAddHintUnlockLeftSeconds(b, int32(s.HintUnlockLeftSeconds))
 	if hasAlphabet {
 		fb.StateViewAddAlphabet(b, alphabet)
 	}