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/querier.go

@@ -25,6 +25,7 @@ type Querier interface {
 	GetEbookRating(ctx context.Context, arg GetEbookRatingParams) (EbookRatings, error)
 	GetEbookRatings(ctx context.Context, ebookID pgtype.UUID) ([]GetEbookRatingsRow, error)
 	GetReadingProgress(ctx context.Context, arg GetReadingProgressParams) (ReadingProgress, error)
+	GetScanSettings(ctx context.Context, id pgtype.UUID) (GetScanSettingsRow, error)
 	GetUser(ctx context.Context, id pgtype.UUID) (GetUserRow, error)
 	GetUserByEmail(ctx context.Context, email string) (Users, error)
 	GetUserByEmailOrUsername(ctx context.Context, email string) (Users, error)
@@ -38,6 +39,7 @@ type Querier interface {
 	UpdateEmail(ctx context.Context, arg UpdateEmailParams) error
 	UpdatePassword(ctx context.Context, arg UpdatePasswordParams) error
 	UpdateReadingProgress(ctx context.Context, arg UpdateReadingProgressParams) (ReadingProgress, error)
+	UpdateScanSettings(ctx context.Context, arg UpdateScanSettingsParams) error
 	UpdateUserTheme(ctx context.Context, arg UpdateUserThemeParams) error
 	UpdateUsername(ctx context.Context, arg UpdateUsernameParams) error
 }