UI: Fix the QML engine instant coding feature

[Alxiice]

Description

Fix the QML engine instant coding feature

[gemini-code-assist]

Code Review

This pull request refactors the QML engine setup in Meshroom. Specifically, QmlInstantEngine in meshroom/ui/utils.py has been refactored to inherit from QObject instead of QQmlApplicationEngine directly, wrapping an internal engine instance and introducing a setupEngine callback to cleanly handle initial loads and hot reloads. meshroom/ui/app.py has been updated to adapt to this new architecture. The review feedback highlights a critical bug in setupEngine where accessing self.engine instead of the passed engine parameter will raise an AttributeError. Additionally, the feedback suggests optimizing engine initialization, increasing the debounce timer interval to properly handle rapid file system events, and robustly re-adding watched files to prevent file-watching loss during 'safe saves'.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.07%. Comparing base (1d89d80) to head (4b61d40).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3156   +/-   ##
========================================
  Coverage    86.07%   86.07%           
========================================
  Files           78       78           
  Lines        12111    12111           
========================================
  Hits         10425    10425           
  Misses        1686     1686           

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[Alxiice]

Comment from #3159

self._debounceTimer = QTimer(parent=self, singleShot=True, interval=100)