fix(security): follow the daemon's lock state on Set-PIN and expiry

Setting a PIN always unlocked the wallet with a 5 s probe and then
re-locked it, even when the wallet was already unlocked, so a PIN check
silently re-locked an unlocked wallet. And once the daemon's absolute
unlock window ran out the GUI still showed "unlocked", so the next send
failed with nothing on screen explaining it.

- The Set-PIN check now decides on the main thread: an unlocked wallet
  is verified with the time it has left (capped at seven days) and is
  not re-locked, so the GUI and the daemon keep agreeing; a locked
  wallet still uses the 5 s probe and re-locks. Either way the daemon's
  lock state is re-read afterwards, which self-heals any drift.
- When the daemon's unlock window has expired but the GUI still says
  "unlocked", a bounded reconcile (once per expiry, 30 s back-off)
  re-reads getwalletinfo so the lock screen and the transaction-cache
  lock follow the daemon. Lite wallets, which have no daemon timer, are
  left alone.

Behaviour change: with auto-lock disabled the daemon still re-locks at
its window, and the GUI now lands on the lock screen instead of
appearing unlocked until a send fails.

Tests: the Set-PIN verify plan (unlocked keeps its remaining time and
skips the re-lock, locked or near-expiry falls back to the 5 s probe and
re-lock, long windows are capped) and the unlock-expiry predicate across
encrypted, locked, no-deadline and not-yet-expired cases.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.h

@@ -1942,6 +1942,8 @@ private:
     // Auto-lock on idle. Fed by real input events (noteUserInput, from main.cpp's event ingest), NOT by
     // widget state: a focused-but-untouched text field must not keep the wallet unlocked (C21).
     util::IdleTracker idle_;
+    // C90: back-off clock for the daemon-unlock-expiry reconcile (steady clock, wall-clock independent).
+    std::chrono::steady_clock::time_point unlock_reconcile_retry_at_{};
 
     // Wall-clock delta for logic timers (see logicDeltaTime()).
     std::chrono::steady_clock::time_point last_update_time_{};
@@ -2052,6 +2054,9 @@ private:
     bool shouldRunWalletTransactionRefresh() const;
     bool shouldRefreshRecentTransactions() const;
     void checkAutoLock();
+    // C90: the daemon's absolute unlock window ran out while the GUI still says "unlocked": re-read getwalletinfo
+    // (once per expiry, backed off) so the lock screen / tx-cache lock follow the daemon.
+    void reconcileUnlockExpiry();
     void checkIdleMining();
 };