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>

tests/test_phase4.cpp

@@ -75,6 +75,7 @@
 #include "util/seed_phrase.h"
 #include "util/local_time.h"
 #include "util/text_fit.h"
+#include "util/idle_tracker.h"
 #include "util/image_decode_core.h"
 #include "daemon/daemon_startup_diagnosis.h"
 #include "util/amount_format.h"
@@ -1132,6 +1133,26 @@ void testSpendableFiltering()
 }
 
 // S8: deferFor() is an absolute hold, independent of the page interval (the retry-storm fix).
+void testIdleTracker()
+{
+    using dragonx::util::IdleTracker;
+    using Clock = IdleTracker::Clock;
+    const auto t0 = Clock::now();
+    IdleTracker idle(t0);
+    EXPECT_NEAR(idle.idleSeconds(t0), 0.0, 1e-9);
+    // No events: idle time keeps growing, however long a (conceptual) field stays focused.
+    EXPECT_NEAR(idle.idleSeconds(t0 + std::chrono::seconds(900)), 900.0, 1e-6);
+    EXPECT_TRUE(idle.idleSeconds(t0 + std::chrono::seconds(901)) >= 900.0);   // would trip a 900 s auto-lock
+    // Activity resets it.
+    idle.noteActivity(t0 + std::chrono::seconds(600));
+    EXPECT_NEAR(idle.idleSeconds(t0 + std::chrono::seconds(900)), 300.0, 1e-6);
+    // Never moves backwards.
+    idle.noteActivity(t0 + std::chrono::seconds(100));
+    EXPECT_NEAR(idle.idleSeconds(t0 + std::chrono::seconds(900)), 300.0, 1e-6);
+    // A query before the last activity is 0, not negative.
+    EXPECT_NEAR(idle.idleSeconds(t0 + std::chrono::seconds(10)), 0.0, 1e-9);
+}
+
 void testRefreshSchedulerDeferFor()
 {
     using dragonx::services::RefreshScheduler;
@@ -17475,6 +17496,7 @@ int main()
     testAmountFormatting();
     testSpendableFiltering();
     testRefreshScheduler();
+    testIdleTracker();
     testRefreshSchedulerDeferFor();
     testRpcBusyGate();
     testIsLongWalletMethod();