feat: create admin dashboard with user preferences and library settings

- Add admin dashboard at /admin route consolidating all user settings
- Move user preferences (username, email, password, theme) from separate page
- Add ebook library preferences section with folder management
- Add scan settings (frequency and auto-scan toggle) with database persistence
- Create database migration for scan_frequency_minutes and auto_scan_enabled columns
- Add API endpoints for scan settings management:
  - PUT /api/library/scan-settings - Update scan preferences
  - GET /api/library/scan-settings - Get current scan settings
- Update dashboard navigation to link to admin dashboard
- Remove old preferences.html template (functionality moved to admin)
- Create Bruno API testing files for library endpoints
- Add real-time folder loading and management in admin interface
- Implement scan settings persistence and retrieval from database

internal/database/queries/queries.sql

@@ -84,6 +84,12 @@ UPDATE users SET password_hash = $2, updated_at = NOW() WHERE id = $1;
 -- name: DeleteUser :exec
 DELETE FROM users WHERE id = $1;
 
+-- name: UpdateScanSettings :exec
+UPDATE users SET scan_frequency_minutes = $2, auto_scan_enabled = $3, updated_at = NOW() WHERE id = $1;
+
+-- name: GetScanSettings :one
+SELECT scan_frequency_minutes, auto_scan_enabled FROM users WHERE id = $1;
+
 -- name: CreateEbookRating :one
 INSERT INTO ebook_ratings (ebook_id, user_id, rating)
 VALUES ($1, $2, $3)