[MEDIA-022] Add a native Nextcloud destination folder picker

[veryCrunchy]

Roadmap

• Task ID: MEDIA-022
• Priority: P0
• Depends on: MEDIA-001

Problem

Media sync setup currently asks users to type a Nextcloud path. Destination selection should use the same native folder navigation model as Files.

Acceptance criteria

• The sync proposal opens a native, folder-only Nextcloud browser instead of requiring path entry.
• The suggested destination is preselected and visible in breadcrumbs.
• Users can navigate, search, create a folder, select the Files root, and confirm without leaving the app.
• Folder loading uses the shared cached Files repository and preserves account context.
• Selection returns a canonical path and prevents choosing an invalid or inaccessible destination.
• Advanced manual path entry remains optional rather than the primary flow.