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/models.go

@@ -55,11 +55,13 @@ type UserEbookFolders struct {
 }
 
 type Users struct {
-	ID           pgtype.UUID        `db:"id" json:"id"`
-	Email        string             `db:"email" json:"email"`
-	Username     string             `db:"username" json:"username"`
-	PasswordHash string             `db:"password_hash" json:"password_hash"`
-	Theme        pgtype.Text        `db:"theme" json:"theme"`
-	CreatedAt    pgtype.Timestamptz `db:"created_at" json:"created_at"`
-	UpdatedAt    pgtype.Timestamptz `db:"updated_at" json:"updated_at"`
+	ID                   pgtype.UUID        `db:"id" json:"id"`
+	Email                string             `db:"email" json:"email"`
+	Username             string             `db:"username" json:"username"`
+	PasswordHash         string             `db:"password_hash" json:"password_hash"`
+	Theme                pgtype.Text        `db:"theme" json:"theme"`
+	CreatedAt            pgtype.Timestamptz `db:"created_at" json:"created_at"`
+	UpdatedAt            pgtype.Timestamptz `db:"updated_at" json:"updated_at"`
+	ScanFrequencyMinutes pgtype.Int4        `db:"scan_frequency_minutes" json:"scan_frequency_minutes"`
+	AutoScanEnabled      pgtype.Bool        `db:"auto_scan_enabled" json:"auto_scan_enabled"`
 }