[Client] Detect main-thread event loop stalls (#7155)

* [Client] Detect main-thread event loop stalls

LagMonitor ticks the GUI event loop every 500 ms and records gaps
beyond 2 s as stalls, warning with their duration and keeping a
bounded ring of recent records for diagnostics. Measurement uses a
monotonic QElapsedTimer so wall-clock steps and suspend do not
fabricate stalls. Recorded timestamps stay in wall time for
correlating with user reports.

Took 1 minute

Took 13 minutes


Took 2 minutes

* [Client] Rename LagMonitor constants to SCREAMING_SNAKE_CASE

Took 15 minutes

* [Client] Discard suspend-spanning gaps in LagMonitor

Windows counts sleep time in its monotonic clock, so a suspend would
fabricate one bogus stall per resume. Reset the clock on application
state changes and drop implausibly huge gaps; extract recordGap() for
testability.

Took 3 minutes

* [Client] Unit test LagMonitor stall recording

Drives recordGap() directly to cover the threshold, plausibility cap,
trim, and clear behavior without timing-dependent waits.

Took 36 seconds

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/src/interface/window_main.h

@@ -25,6 +25,7 @@
 #ifndef WINDOW_H
 #define WINDOW_H
 
+#include "../client/lag_monitor.h"
 #include "connection_controller/remote_connection_controller.h"
 #include "widgets/dialogs/dlg_local_game_options.h"
 
@@ -145,6 +146,7 @@ private:
     WndSets *wndSets;
     ConnectionController *connectionController;
     LocalServer *localServer;
+    LagMonitor lagMonitor; ///< watches the main thread for event loop stalls
     bool bHasActivated, askedForDbUpdater;
     QProcess *cardUpdateProcess;
     DlgViewLog *logviewDialog;