feat: add Obsidian export with path traversal protection

[viperrcrypto]

Summary

• Adds Obsidian vault export (Markdown notes with YAML frontmatter, wikilinks, index files)
• Fixes path traversal vulnerability from original PR Feat: Add basic export flow for Obsidian. #42 by adding validateVaultPath()
• Rebased onto current main (resolves conflicts with PRs Add MiniMax as third AI provider #57, feat: add download as markdown button to bookmark cards #61, Add compact list view mode to bookmarks page #37, upgraded pagination with jump to page, page counter, first, prev, next and last page buttons #38, fix: bookmarklet capture broken by X API changes #68)

Supersedes #42.

Security fixes applied

• validateVaultPath() blocks system directories (/etc, /usr, etc.)
• Requires absolute path to an existing directory
• Sanitizes subfolder param and verifies resolved path stays under vault
• Export route doesn't leak internal error messages

Test plan

• Type check passes
• No conflicts with current main
• Save vault path in settings - verify validation rejects /etc, /tmp
• Export to valid Obsidian vault - verify notes created with frontmatter
• Verify index files generated for categories and authors

Co-Authored-By: KauriHero 55721017+kauri-hero@users.noreply.github.com

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