feat: add Milkdown as default WYSIWYG editor

[tmad4000]

Summary

• Adds Milkdown as the default rich editor engine (true WYSIWYG)
• Keeps EasyMDE as an alternative option for users who prefer markdown preview style
• Adds "Rich Editor Engine" submenu in Settings to switch between editors

Changes

• Pre-bundled Milkdown with esbuild for reliable dependency resolution
• Added Milkdown-specific styles matching app's dark/light theme
• Editor engine preference saved to config and restored on launch

How it works

Milkdown provides a true WYSIWYG experience where formatting is rendered inline (bold text appears bold, headers are large, etc.), while EasyMDE shows markdown syntax with styling hints. Users can switch between them via Settings > Rich Editor Engine.

Test plan

• Open a markdown file and enter edit mode
• Verify Milkdown WYSIWYG editor loads by default
• Type some markdown and verify formatting renders inline
• Switch to EasyMDE via Settings > Rich Editor Engine > EasyMDE
• Verify content syncs correctly between editors
• Restart app and verify editor preference is persisted

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