fix(core): drive auto-lock idle from real input, not widget focus

The auto-lock idle clock was reset whenever any ImGui item was active,
so a text field left focused (a half-filled send form, say) counted as
continuous activity and kept the wallet unlocked indefinitely, well past
the configured idle timeout.

- The idle clock is now fed by real input events (key, text, mouse,
  touch, pen, gamepad button) as they are ingested on the main thread,
  plus a still-held mouse button so a drag or scrollbar grab that emits
  no new event still counts. A focused but untouched widget no longer
  does.
- A new header-only IdleTracker owns the clock and never moves backwards,
  so an event stamped before the previous one cannot shorten the idle
  time.

Behaviour change: a form left focused and idle past the timeout now
auto-locks. The form's buffers are app state and survive the lock and
unlock, so nothing typed is lost.

Tests: a new IdleTracker case covers idle time growing with no events
(enough to trip a 900s lock), activity resetting it, the clock never
moving backwards, and a query before the last activity reading zero
rather than a negative value.

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

src/app_security.cpp

@@ -479,7 +479,7 @@ void App::applyUnlockSuccess(const std::string& passphrase, int timeout) {
     lock_error_msg_.clear();
     lock_attempts_ = 0;
     memset(lock_passphrase_buf_, 0, sizeof(lock_passphrase_buf_));
-    last_interaction_ = std::chrono::steady_clock::now();
+    idle_.noteActivity();
     state_.encrypted = true;
     state_.locked = false;
     state_.unlocked_until = std::time(nullptr) + timeout;
@@ -726,17 +726,16 @@ void App::checkAutoLock() {
                         || (xmrig_manager_ && xmrig_manager_->isRunning());
     if (miningActive) {
         // Keep resetting the idle timer so we don't lock the instant mining stops
-        last_interaction_ = std::chrono::steady_clock::now();
+        idle_.noteActivity();
         return;
     }
 
     int timeout = settings_ ? settings_->getAutoLockTimeout() : 300;
     if (timeout <= 0) return; // disabled
 
-    auto now = std::chrono::steady_clock::now();
-    float elapsed = std::chrono::duration<float>(now - last_interaction_).count();
+    const double elapsed = idle_.idleSeconds();
 
-    if (elapsed >= (float)timeout) {
+    if (elapsed >= (double)timeout) {
         // Lite has no daemon `walletlock` — App::lockWallet() early-returns without rpc_, so route
         // lite through lockLiteWallet() (locks the backend + tears down the chat session so no
         // decrypted store / unlocked DB key survives the idle lock). In full-node builds