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.

docs/FUNCTIONAL.md

@@ -220,11 +220,11 @@ personal hint wallet once the per-game allowance is spent. Against the robot
 anti-frustration aid: one unlocks only after the player has been **stuck ~30 minutes
 on a turn** (timed from the robot's last move; the very first move, before the robot
 has played, is exempt). While gated the hint button carries a small **🔒 lock** and a
-tap shows how long remains; the lock lifts live at the mark. The wait **persists across
-leaving and reopening the app**, so a stuck turn is not forgotten. It stays robust to a
-device clock change: the remaining is capped at the window, so a clock set back cannot
-freeze it, and a clock set forward merely opens the hint early — harmless in a solo game.
-The game ends when the
+tap shows how long remains; the lock lifts live at the mark. The same gate applies **online and
+offline**: the countdown runs on a **steady in-app timer**, not the device clock, so changing the
+clock cannot skew it, and a relaunch re-reads the remaining time so a stuck turn is not forgotten.
+Online the **server enforces** it from its own clock (which the player cannot touch); a vs_ai hint
+counts toward no hint statistic. The game ends when the
 bag empties and a player clears their rack, after 6 consecutive scoreless turns,
 by resignation, or by the per-game move timeout (5 minutes to 24 hours, default
 24 hours): a missed turn auto-resigns, except while the player is inside their