feat(hint): idle-gated wallet-free vs_ai hint on a monotonic clock (B4)

A vs_ai hint is now unlimited and wallet-free, but idle-gated as an anti-frustration
aid: it unlocks only after the player has been stuck ~30 min on a turn (timed from the
robot's last move; the human's first move, before the robot has played, is exempt).
While gated the hint button carries a small lock badge and a tap shows the remaining
minutes ('Available in N min.'); the lock lifts live at the mark.

The gate is enforced CLIENT-SIDE against a MONOTONIC clock (performance.now()), never a
wall-clock timestamp: a device clock the player controls (or an auto-sync) must not be
able to open or freeze it. The wait is therefore session-scoped -- a reload restarts it
(there is no tamper-proof way to carry idle time across a relaunch without a wall clock).
An online vs_ai game will gate the same way but from the server's clock (a follow-up).

- lib/hints.ts: HINT_GATE_MS + pure hintGateRemainingMs (monotonic) + hintLockMinutes;
  removed the wall-clock hintRemainingMs. Unit-tested red->green.
- Game.svelte: the monotonic gate (hintGateStart/monoNow, armed on the turn change), a
  vs_ai hint button (plain: no confirm, no count; lock badge + gated tap -> toast), a
  live 10s tick.
- localgame: removed the wall-clock hint gate and the now-dead robotLastMoveAtUnix field
  from source.ts/serialize.ts (the client is authoritative); hint() just serves the top
  move.
- i18n game.hintLockedIn.
- offline.spec.ts: assert the first move is un-gated and the lock arms after the robot moves.
- Docs FUNCTIONAL(+_ru) + ARCHITECTURE; bundle budget 114->115 (game-screen feature).

docs/ARCHITECTURE.md

@@ -1252,7 +1252,11 @@ an *Offline* chip and confines play to on-device `vs_ai` games. The **offline lo
 device-local games** (reconstructed by replaying the IndexedDB move journal) and its New-vs-AI entry
 creates one through the in-browser engine — the same game screen then drives it, the robot replying
 locally; online-only affordances (the Stats tab, the friend/random options in New Game) are disabled
-or hidden. To have data ready before the switch, the **Profile advertises the current dictionary
+or hidden. A local `vs_ai` hint is unlimited and wallet-free but idle-gated (unlocked ~30 min into a
+stuck turn); the gate is enforced **client-side against a monotonic clock** (`performance.now()`,
+`lib/hints`), never a wall-clock timestamp — a device clock the player controls must not be able to
+open or freeze it, so the wait is session-scoped (a reload restarts it). An online `vs_ai` game will
+gate the same way but from the server's clock (a follow-up). To have data ready before the switch, the **Profile advertises the current dictionary
 version per variant** (`dict_versions`,
 filled from the registry on the existing cold-start profile request — no extra round-trip), and an
 eligible installed PWA (standalone web + confirmed email) **background-preloads** those dictionaries