feat(canvas): Delegate flask_editor to figrecipe._editor

[ywatanabe1989]

Summary

Delegate scitex's canvas/editor/flask_editor to figrecipe._editor as single source of truth.

Current State

• scitex/canvas/editor/flask_editor/ has ~10,000+ lines of code
• figrecipe/_editor/ has the same Flask-based figure editor
• This is duplicated functionality

Proposed Solution

Follow the same pattern as diagram and plt delegation:

1. Remove scitex's flask_editor implementation
2. Import and re-export from figrecipe._editor
3. Keep scitex.canvas as thin wrapper

Benefits

• Single source of truth (figrecipe)
• Reduced maintenance burden
• Consistent with diagram/plt delegation pattern

Related Issues

• Closes Refactor: _scripts.py (4933 lines → <512) #96 (_scripts.py refactoring - now just deleted)
• Closes Refactor: flask_editor/_core.py (1688 lines → <512) #102 (flask_editor/_core.py refactoring)
• Closes Refactor: _styles.py (1658 lines → <512) #103 (_styles.py refactoring - now just deleted)

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