feat(hint): persist the vs_ai idle-hint wait (wall-clock + read sanitiser)

Per the owner's call, the idle-hint gate now PERSISTS across leaving and reopening the
app, instead of the session-scoped monotonic clock: the unlock is a wall-clock instant
(hintUnlockAtMs) stamped from the robot's reply, stored on the local record, carried on
the game view + the opponent_moved move delta, and read through a sanitiser that caps it
at now + the window. So:
- the wait survives a relaunch (a stuck turn is not forgotten);
- a device clock set BACK cannot freeze the gate (the cap bounds the remaining to the
  window and self-heals on the next read);
- a clock set FORWARD just opens the hint early -- accepted as harmless for a solo game.

- lib/hints.ts hintGateRemainingMs now takes the unlock instant + clamps to the window.
- localgame: re-add hintUnlockAtMs to the record/meta; stamp it off the robot's reply;
  sanitise on read (a shared hintUnlock helper feeds stateView + the opponent_moved event).
- gamedelta + PushEvent: the move delta carries hintUnlockAtMs so the view stays fresh.
- Game.svelte: hintUnlockAt derives from the view; the tick + lock + toast unchanged.
- offline.spec.ts: also assert the lock survives a reload (the wait persisted).
- Docs FUNCTIONAL(+_ru)/ARCHITECTURE updated (persist + sanitiser, not monotonic-resets).

ui/src/lib/model.ts

@@ -82,6 +82,10 @@ export interface StateView {
   bagLen: number;
   hintsRemaining: number;
   walletBalance: number;
+  /** For a vs_ai game, the wall-clock ms at which the idle hint unlocks — persisted so the wait
+   *  survives a relaunch, and sanitised on read so a device clock set back cannot freeze it — or
+   *  0/undefined while open (the human's first move, or a non-vs_ai game). See lib/hints. */
+  hintUnlockAtMs?: number;
 }
 
 export interface MoveResult {
@@ -421,7 +425,7 @@ export interface GameList {
  */
 export type PushEvent =
   | { kind: 'your_turn'; gameId: string; deadlineUnix: number; opponentName: string; moveCount: number }
-  | { kind: 'opponent_moved'; gameId: string; move?: MoveRecord; game?: GameView; bagLen: number }
+  | { kind: 'opponent_moved'; gameId: string; move?: MoveRecord; game?: GameView; bagLen: number; hintUnlockAtMs?: number }
   | { kind: 'game_over'; gameId: string; result: string; scoreLine: string; game?: GameView }
   | { kind: 'chat_message'; message: ChatMessage }
   | { kind: 'nudge'; gameId: string; fromUserId: string; senderName: string }