feat: add Markdown export feature for tasks (#869)

[Xenon010101]

Adds a downloadable Markdown export of all active tasks, compatible with Obsidian, Notion, Logseq, and any Markdown editor.

Changes

• \exportToMarkdown()\ JS function: formats tasks as grouped checklists organized by subject, sorted by deadline
• 'Export to MD' button added to the dashboard tasks-actions-bar
• Exports \StudyPlan_Export.md\ with clean Markdown checklist format
• Tasks show as [x]\ (completed) or [ ]\ (pending) with title, deadline, notes
• Fully client-side: no backend changes, no database queries — pure Blob + <a>\ download
• Shows toast notification on success/error

Closes #869