feat(web): add sidebar with search, filters, and folder tree to media library

[devin-ai-integration]

feat(web): add sidebar with search, filters, and folder tree to media library

Summary

Adds a collapsible left sidebar to the media library with search, file type filtering, and folder tree navigation. This is PR #6 in the stacked PR series for issue #2948 (new blog admin site).

New features:

• Search input that filters files by name (case-insensitive)
• File type filter buttons: All Files, Images, Videos, Documents
• Folder tree navigation with expandable nodes (fetches up to 3 levels deep)
• Collapsible sidebar with toggle button in header
• "Clear Filters" button when filters are active
• Empty state message when no files match filters

Layout changes:

• Restructured from single column to flex layout with sidebar + main content
• Sidebar is 256px wide (w-64) or collapsed to 0
• Main content area scrolls independently

Review & Testing Checklist for Human

• Verify modal positioning - The layout restructuring moved modals inside nested divs. Test that rename dialog, move dialog, and context menu still display correctly with proper z-index layering
• Test folder tree with deep nesting - The recursive fetch goes 3 levels deep. Verify it doesn't cause performance issues or excessive API calls with many folders
• Test search filtering - No debouncing implemented, so verify it doesn't lag with many files
• Verify sidebar collapse/expand - Test the transition animation and that content reflows correctly

Recommended test plan:

1. Navigate to /admin/media in the preview deployment
2. Toggle sidebar collapse/expand using the hamburger menu button
3. Type in search box and verify files filter in real-time
4. Click each file type filter and verify correct filtering
5. Click "Clear Filters" and verify it resets
6. Expand folders in the tree and click to navigate
7. Right-click a file to verify context menu still works
8. Test rename and move dialogs to verify they display correctly over the sidebar

Notes

This is PR #6 in a stacked PR series. It depends on PR #5 (context menu & item actions).

Link to Devin run: https://app.devin.ai/sessions/5c615647f58c421dabccc0cdd5b4270c
Requested by: @ComputelessComputer

[devin-ai-integration]

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