feat: add directory filter dropdown to dashboard sessions panel

[sleipi]

Summary

• Add a dropdown filter in the right panel of the dashboard to filter sessions by working directory
• New getDistinctDirectories() repo method returns all unique directory values for populating the dropdown
• getRootSessions() accepts an optional directory parameter to filter results
• Dropdown uses custom styling (no native browser chrome) matching the dark dashboard theme
• Filter state persists across auto-refresh cycles via JS save/restore
• New /api/directories JSON endpoint for programmatic access
• 3 new unit tests covering directory filtering, deduplication, and null exclusion