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.cpp

@@ -868,14 +868,17 @@ void App::update()
     // If a wallet switch's daemon failed to start, revert to the previous wallet (main-thread-safe).
     processWalletSwitchRevert();
 
-    // Track user interaction for auto-lock. Not in the background: a minimized window runs update() without
-    // new frames, so the IO state is the last frame's, and a stale mouse delta kept auto-lock from ever firing.
+    // Real input arrives through App::noteUserInput() (main.cpp event ingest). What stays here covers input
+    // that produces no new SDL event this frame: a held mouse button (a drag or scrollbar grab without
+    // motion). NOT in the background: a minimized window runs update() with the last frame's IO, and a stale
+    // mouse delta once kept auto-lock from ever firing. Deliberately NOT IsAnyItemActive(): a focused text
+    // field stays active indefinitely (C21).
     if (!window_background_ &&
         (io.MouseDelta.x != 0 || io.MouseDelta.y != 0 ||
          io.MouseClicked[0] || io.MouseClicked[1] ||
          io.InputQueueCharacters.Size > 0 ||
-         ImGui::IsAnyItemActive())) {
-        last_interaction_ = std::chrono::steady_clock::now();
+         ImGui::IsAnyMouseDown())) {
+        idle_.noteActivity();
     }
 
     // Advance the refresh timers by the real time since the last update BEFORE the RPC results are