Allow drag & drop files on InitNode in the GraphEditor

[Copilot]

• Understand why GraphEditor's DropArea intercepts all drop events (higher z-order than nodes inside draggable)
• Remove the DropArea from Node.qml; add initNodeDragHover property and highlight rectangle
• Add initNodeDelegateAt() helper function to GraphEditor.qml
• Update GraphEditor's DropArea to detect InitNode under cursor and call initializeNode on drop
• Add onPositionChanged/onExited to GraphEditor's DropArea for visual feedback

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.39%. Comparing base (bde64b0) to head (67f1009).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
meshroom/core/node.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3068      +/-   ##
===========================================
- Coverage    83.40%   83.39%   -0.01%     
===========================================
  Files           73       73              
  Lines         9870     9873       +3     
===========================================
+ Hits          8232     8234       +2     
- Misses        1638     1639       +1     

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[cbentejac]

This implementation always re-initializes the InitNode when files/images are dropped through the initialize() method. Shouldn't we also want to be able to extend the current input with the files/images we are dropping?

We could assume that the current behaviour is the default one, but if files/images are dropped while pressing a specific key, then the behaviour changes to an extendAttributes() instead of an initalize().

[cbentejac]

Suggested change

	if not isinstance(node.nodeDesc, meshroom.core.desc.InitNode):
	if not node.isInitNode: