feat: add backup start time configuration and theme switch functionality

- Introduced BACKUP_DEFAULT_START_TIME constant for backup scheduling.
- Updated BackupScheduleConfig interface to include startTime.
- Implemented normalizeStartTime function for validating and normalizing start time input.
- Enhanced backup settings parsing to accommodate start time.
- Added start time input field in BackupDestinationDetail component.
- Created ThemeSwitch component for toggling between light and dark themes.
- Integrated theme preference management in App component.
- Updated styles for dark mode support across the application.
- Added translations for theme toggle and backup start time labels.

shared/backup-schema.ts

@@ -3,6 +3,7 @@ export const BACKUP_DEFAULT_RETENTION_COUNT = 30;
 export const BACKUP_DEFAULT_E3_REGION = 'auto';
 export const BACKUP_DEFAULT_REMOTE_PATH = 'nodewarden';
 export const BACKUP_DEFAULT_INTERVAL_HOURS = 24;
+export const BACKUP_DEFAULT_START_TIME = '03:00';
 
 export type BackupDestinationType = 'e3' | 'webdav';
 
@@ -40,6 +41,7 @@ export interface BackupRuntimeState {
 export interface BackupScheduleConfig {
   enabled: boolean;
   intervalHours: number;
+  startTime: string;
   timezone: string;
   retentionCount: number | null;
 }
@@ -82,6 +84,7 @@ export function createDefaultBackupScheduleConfig(timezone: string = BACKUP_DEFA
   return {
     enabled: false,
     intervalHours: BACKUP_DEFAULT_INTERVAL_HOURS,
+    startTime: BACKUP_DEFAULT_START_TIME,
     timezone,
     retentionCount: BACKUP_DEFAULT_RETENTION_COUNT,
   };