Fix camera malfunction crashes by implementing QThread-based solution

[barpavel]

PROBLEM:
Application crashed/froze when camera malfunctioned because camera I/O operations were blocking the main UI thread using QTimer.

SOLUTION:
In order to eliminate application freezes when camera malfunctions, I replaced an existing timer-based camera capture (QTimer approach) with a dedicated background thread (a VideoThread class) that:

• Inherits from QThread to run camera operations in background thread.
• Uses pyqtSignal for thread-safe communication with UI thread.
• Implements proper thread life-cycle management:
  • start()
  • stop()
  • cleanup()
• Provides robust error handling and user feedback.

CORE CHANGES:

• Replaced self.timer (a QTimer object) with self.video_thread (a VideoThread object).
• Moved all camera operations (cv2.VideoCapture(), cap.read(), etc.) to the new background thread.
• Implemented signal-based communication for thread-safe frame/error handling.
• Added comprehensive error handling with user-friendly messages.

TECHNICAL DETAILS:

• Added VideoThread class (QThread subclass) for background camera capture.
• Removed QTimer-based view_video() method that blocked main thread and unused QTimer module import.
• Removed unnecessary time.sleep(0.1) camera initialization delay and unused time module import.
• Removed global timer & cap variables (unsafe across threads) and an ugly globals() usage together with a pending TODO: comment.
• Signal-based communication: frame_captured and error_occurred signals:
  • Thread-safe frame updates via on_frame_captured() slot.
  • Graceful error handling via on_video_error() slot.
• Additionally performed some refactoring, i.e.:
  • Modifying a _read_video_frame() static method into a read_frame() inner method of the main thread execution run() method, which results in a more clean and Pythonic code.
  • Updated the error messages to clearly distinguish between initialization failures (the camera/video cannot be opened initially) and runtime failures (the camera/video was working but stopped).

BENEFITS:

• UI remains responsive during camera errors (no more freezing).
• Graceful handling of hardware failures (no more crashes).
• Professional error messages displayed to the user.
• Follows Qt best practices for hardware I/O.

Resolves: #5

Co-authored-by: Cursor AI
Signed-off-by: Pavel Bar pbar@redhat.com

[barpavel]

@SihabSahariar I implemented the QThread solution in the app.py and temporary added a QThread_design folder with solution documentation.
If you want to read some documentation explaining the solution, you can start reading from DOCUMENTATION_INDEX.md.
When you review the solution (and if possible test the scenarios that previously crashed the application) I will remove the folder. Or maybe I can attach it as zip file to this PR just for history, but not part of the repo.

[barpavel]

@SihabSahariar Actually I planned to remove QThread_design folder with solution documentation after your code review.
I doubt it should be in the repo.