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/handlers/auth.go

@@ -588,6 +588,62 @@ func (h *AuthHandler) DeleteAccount(c echo.Context) error {
 	return c.JSON(http.StatusOK, map[string]string{"message": "account deleted successfully"})
 }
 
+type UpdateScanSettingsRequest struct {
+	ScanFrequencyMinutes int32 `json:"scan_frequency_minutes" validate:"required,min=15,max=1440"`
+	AutoScanEnabled      bool  `json:"auto_scan_enabled"`
+}
+
+// UpdateScanSettings handles PUT /api/library/scan-settings
+func (h *AuthHandler) UpdateScanSettings(c echo.Context) error {
+	userID := c.Get("user_id").(string)
+	userUUID, err := uuid.Parse(userID)
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid user id"})
+	}
+
+	var req UpdateScanSettingsRequest
+	if err := c.Bind(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request"})
+	}
+	if err := c.Validate(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
+	}
+
+	err = h.db.UpdateScanSettings(c.Request().Context(), database.UpdateScanSettingsParams{
+		ID:                   pgtype.UUID{Bytes: userUUID, Valid: true},
+		ScanFrequencyMinutes: pgtype.Int4{Int32: req.ScanFrequencyMinutes, Valid: true},
+		AutoScanEnabled:      pgtype.Bool{Bool: req.AutoScanEnabled, Valid: true},
+	})
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	return c.JSON(http.StatusOK, map[string]string{"message": "scan settings updated successfully"})
+}
+
+// GetScanSettings handles GET /api/library/scan-settings
+func (h *AuthHandler) GetScanSettings(c echo.Context) error {
+	userID := c.Get("user_id").(string)
+	userUUID, err := uuid.Parse(userID)
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid user id"})
+	}
+
+	settings, err := h.db.GetScanSettings(c.Request().Context(), pgtype.UUID{Bytes: userUUID, Valid: true})
+	if err != nil {
+		// If no settings found, return defaults
+		return c.JSON(http.StatusOK, map[string]interface{}{
+			"scan_frequency_minutes": 60,
+			"auto_scan_enabled":      true,
+		})
+	}
+
+	return c.JSON(http.StatusOK, map[string]interface{}{
+		"scan_frequency_minutes": settings.ScanFrequencyMinutes.Int32,
+		"auto_scan_enabled":      settings.AutoScanEnabled.Bool,
+	})
+}
+
 func (h *AuthHandler) generateJWT(userID string) (string, error) {
 	claims := jwtgo.MapClaims{
 		"user_id": userID,