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.

ui/src/lib/gamedelta.ts

@@ -11,9 +11,6 @@ export interface MoveDelta {
   move?: MoveRecord;
   game?: GameView;
   bagLen: number;
-  /** For a vs_ai game, the refreshed idle-hint unlock instant (the robot's reply re-arms the gate),
-   *  so the cached view stays current without a refetch; absent leaves the prior value. */
-  hintUnlockAtMs?: number;
 }
 
 /**
@@ -53,12 +50,7 @@ export function applyMoveDelta(cached: CachedGame | undefined, d: MoveDelta): De
   if (next > have + 1) return { refetch: true }; // a gap — an intermediate move was missed
   // The actor's own move changed their rack (a draw), which opponent_moved does not carry.
   if (d.move.player === cached.view.seat) return { refetch: true };
-  const view: StateView = {
-    ...cached.view,
-    game: d.game,
-    bagLen: d.bagLen,
-    hintUnlockAtMs: d.hintUnlockAtMs ?? cached.view.hintUnlockAtMs,
-  };
+  const view: StateView = { ...cached.view, game: d.game, bagLen: d.bagLen };
   return { cache: { view, moves: [...cached.moves, d.move] }, refetch: false };
 }
 
@@ -87,7 +79,7 @@ export function applyGameOver(cached: CachedGame | undefined, game: GameView | u
  */
 export function advanceCached(cached: CachedGame | undefined, e: PushEvent): CachedGame | undefined {
   if (e.kind === 'opponent_moved') {
-    return applyMoveDelta(cached, { move: e.move, game: e.game, bagLen: e.bagLen, hintUnlockAtMs: e.hintUnlockAtMs }).cache;
+    return applyMoveDelta(cached, { move: e.move, game: e.game, bagLen: e.bagLen }).cache;
   }
   if (e.kind === 'game_over') return applyGameOver(cached, e.game).cache;
   if (e.kind === 'opponent_joined' && e.state) return applyOpponentJoined(cached, e.state);