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/main.cpp

@@ -1605,7 +1605,10 @@ int main(int argc, char* argv[])
     Uint64 lastFrameStartNs = 0;     // software frame cap bookkeeping (SDL_GetTicksNS)
     int settleFrames = kSettleFrames;
     auto noteEvent = [&](const SDL_Event& e) {
-        if (isInputEvent(e.type)) lastInputMs = SDL_GetTicks();
+        if (isInputEvent(e.type)) {
+            lastInputMs = SDL_GetTicks();
+            app.noteUserInput();   // auto-lock idle clock (C21): real input only, never widget state
+        }
         else if (isWindowStateEvent(e.type)) settleFrames = kSettleFrames;
     };