Add note export functionality to web app

[typelets]

Description:
Users should be able to export their notes from the web app for backup, migration, or offline access. This feature will allow exporting individual notes or bulk exporting all notes.

Feature Request:

Phase 1 (MVP):

• Single note export as Markdown (.md)
• Add "Export" option to note menu (⋮)
• Client-side download (no server processing)
• Convert TipTap HTML content to Markdown using Turndown

Phase 2 (Bulk Export):

• Export all notes as ZIP file
• Preserve folder structure in export
• Add "Export All Notes" option in Settings page
• Use JSZip library for ZIP creation

Phase 3 (Advanced):

• Multiple format support (Markdown, HTML, Plain Text, PDF)
• Folder-level export (export specific folder and its contents)
• Selective export with checkbox selection mode
• Export encrypted backup (for restore purposes)

UI/UX:

• Single note: Add to note menu (⋮) → "Export"
• Bulk export: Settings → Data & Privacy → "Export All Notes"
• Format selector dropdown (Markdown, HTML, TXT, PDF)
• Progress indicator for large exports

Benefits:

• Data portability and ownership
• Backup and disaster recovery
• Migration to other platforms
• Offline access to notes
• Compliance with data export regulations

Platform:
Web app

Labels: enhancement, web, feature-request, data-export