Add asset management tab with file-explorer view

[yusufipk]

Summary

Add a project-level Asset Management tab that lets users browse all videos and assets in a file-explorer-style view, with the ability to preview, filter, and delete items without opening each video individually.

Current behavior

• Videos are listed on the project page as cards; each must be opened to manage it.
• Assets are scoped to a single video in the per-video Assets pane (components/video-page/assets-pane.tsx).
• Deleting an asset requires navigating to that video's page and deleting from the asset list.
• There is no unified view of all media across a project.

Expected behavior

• New tab or section on the project page (e.g. Assets or Media) alongside the existing video grid.
• File-explorer-style layout:
  • Folder/tree or grouped list by video (and optionally by type: video versions, images, audio, YouTube links, Bunny uploads).
  • Columns or list rows showing name, type, size/duration where available, upload date, and uploader.
  • Search and filters (by type, video, date, name).
• Actions per item and in bulk:
  • Delete individual assets or videos.
  • Multi-select + bulk delete.
  • Download where already supported.
• Inline preview where practical (thumbnail for images, basic info for other types).
• Clear empty states and permission gating (same rules as today: owners/admins manage; viewers read-only).

Technical notes

• Asset APIs exist per video: GET/DELETE /api/videos/[videoId]/assets/[assetId].
• Video delete API: DELETE /api/projects/[projectId]/videos/[videoId].
• May need a new project-scoped listing endpoint (e.g. aggregate assets across videos in a project) for efficient pagination rather than N+1 fetches per video.
• Reuse existing cleanup paths for Bunny stream and R2 on delete.

Acceptance criteria

• Project page has a dedicated asset/media management view accessible without opening each video
• User can browse all project videos and their assets in a list or tree layout
• User can delete individual assets from this view
• User can delete videos from this view (with confirmation)
• Bulk selection and bulk delete supported for assets and/or videos
• Search or filter helps find items in projects with many videos/assets
• Permissions match existing single-item delete rules