docs(api): document library folder browsing endpoint

- Add docs/developer/api/libraries/browse-folders.md
- Document security features (path traversal protection, admin-only)
- Include usage examples and error responses
- Clarify browses server filesystem, not client's

docs(user): add admin library folder browser documentation

- Add docs/user/admin-library-management.md
- Document how to use the folder browser UI
- Include security notes and manual entry instructions
- Explain server filesystem browsing behavior

Fixes: Issue 2 (documentation)

docs/user/admin-library-management.md

@@ -0,0 +1,32 @@
+# Admin Library Management
+
+## Adding Library Folders
+
+When creating or managing a library, you can add folders containing your media files (ebooks, comics, manga).
+
+### Using the Folder Browser
+
+The admin library page includes a folder browser to help you select folders on the server:
+
+1. Navigate to **Admin → Library Management**
+2. Find the library you want to manage
+3. Click the **Folders** button
+4. Click **Browse** next to "Add folder path"
+5. Navigate through the server's filesystem
+6. Select a folder by clicking **Select This Folder**
+
+### Security
+
+- The folder browser only shows directories (not files)
+- Path traversal is protected (cannot access parent directories with `..`)
+- Only admin users can browse the filesystem
+
+### Manual Entry
+
+Alternatively, you can manually type the full server path if you know it:
+
+```
+/home/user/books
+/media/external/ebooks
+/var/lib/manga
+```