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/util/idle_tracker.h

@@ -0,0 +1,22 @@
+// Idle tracking for the auto-lock: pure, header-only, main-thread only.
+#pragma once
+#include <chrono>
+
+namespace dragonx {
+namespace util {
+
+class IdleTracker {
+public:
+    using Clock = std::chrono::steady_clock;
+    explicit IdleTracker(Clock::time_point start = Clock::now()) : last_(start) {}
+    // Never moves backwards (an event stamped before the last one must not shorten the idle time).
+    void noteActivity(Clock::time_point t = Clock::now()) { if (t > last_) last_ = t; }
+    double idleSeconds(Clock::time_point now = Clock::now()) const {
+        return now <= last_ ? 0.0 : std::chrono::duration<double>(now - last_).count();
+    }
+private:
+    Clock::time_point last_;
+};
+
+}  // namespace util
+}  // namespace dragonx