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.

src/services/backup-config.ts

@@ -8,6 +8,7 @@ import {
 } from './backup-settings-crypto';
 import {
   BACKUP_DEFAULT_INTERVAL_HOURS,
+  BACKUP_DEFAULT_START_TIME,
   BACKUP_DEFAULT_TIMEZONE,
   type BackupDestinationConfig,
   type BackupDestinationRecord,
@@ -90,6 +91,20 @@ function normalizeIntervalHours(value: unknown, fallback: number = BACKUP_DEFAUL
   return raw;
 }
 
+function normalizeStartTime(value: unknown, fallback: string = BACKUP_DEFAULT_START_TIME): string {
+  const raw = asTrimmedString(value) || fallback;
+  const match = raw.match(/^(\d{1,2})(?::(\d{1,2}))?$/);
+  if (!match) {
+    throw new Error('Backup start time must be in HH:mm format');
+  }
+  const hour = Number(match[1]);
+  const minute = Number(match[2] ?? '0');
+  if (!Number.isInteger(hour) || !Number.isInteger(minute) || hour < 0 || hour > 23 || minute < 0 || minute > 59) {
+    throw new Error('Backup start time must be in HH:mm format');
+  }
+  return `${String(hour).padStart(2, '0')}:${String(minute).padStart(2, '0')}`;
+}
+
 function normalizeE3Destination(value: unknown, allowIncomplete = false): E3BackupDestination {
   const source = isPlainObject(value) ? value : {};
   const endpoint = asTrimmedString(source.endpoint);
@@ -219,6 +234,10 @@ function normalizeDestinationRecord(
       scheduleSource.intervalHours ?? previousSchedule.intervalHours,
       previousSchedule.intervalHours || BACKUP_DEFAULT_INTERVAL_HOURS
     ),
+    startTime: normalizeStartTime(
+      scheduleSource.startTime ?? previousSchedule.startTime,
+      previousSchedule.startTime || BACKUP_DEFAULT_START_TIME
+    ),
     timezone: assertValidTimeZone(asTrimmedString(scheduleSource.timezone ?? previousSchedule.timezone) || fallbackTimezone || BACKUP_DEFAULT_TIMEZONE),
     retentionCount: normalizeRetentionCount(retentionSource, previousSchedule.retentionCount),
   };
@@ -259,6 +278,7 @@ function parseLegacyBackupSettings(rawValue: Record<string, unknown>, fallbackTi
     schedule: {
       enabled: !!rawValue.enabled,
       intervalHours,
+      startTime: BACKUP_DEFAULT_START_TIME,
       timezone: assertValidTimeZone(asTrimmedString(rawValue.timezone) || fallbackTimezone || BACKUP_DEFAULT_TIMEZONE),
       retentionCount: 30,
     },
@@ -326,6 +346,7 @@ export function parseBackupSettings(raw: string | null, fallbackTimezone: string
           schedule: {
             enabled: scheduleEnabled,
             intervalHours: globalIntervalHours,
+            startTime: BACKUP_DEFAULT_START_TIME,
             timezone: globalTimezone,
             retentionCount: 30,
           },
@@ -495,15 +516,87 @@ export function getBackupLocalTime(date: Date, timezone: string): string {
   return `${parts.hour}:${parts.minute}`;
 }
 
+function parseLocalDateKey(dateKey: string): { year: number; month: number; day: number } | null {
+  const match = String(dateKey || '').match(/^(\d{4})-(\d{2})-(\d{2})$/);
+  if (!match) return null;
+  const year = Number(match[1]);
+  const month = Number(match[2]);
+  const day = Number(match[3]);
+  if (!Number.isInteger(year) || !Number.isInteger(month) || !Number.isInteger(day)) return null;
+  return { year, month, day };
+}
+
+function getUtcDateForLocalTime(timezone: string, year: number, month: number, day: number, hour: number, minute: number): Date {
+  const utcGuess = Date.UTC(year, month - 1, day, hour, minute, 0, 0);
+  const actual = getDateTimeParts(new Date(utcGuess), timezone);
+  const actualUtc = Date.UTC(
+    Number(actual.year),
+    Number(actual.month) - 1,
+    Number(actual.day),
+    Number(actual.hour),
+    Number(actual.minute),
+    0,
+    0
+  );
+  const desiredUtc = Date.UTC(year, month - 1, day, hour, minute, 0, 0);
+  return new Date(utcGuess - (actualUtc - desiredUtc));
+}
+
+function getBackupSlotStartsForLocalDay(
+  dateKey: string,
+  timezone: string,
+  startTime: string,
+  intervalHours: number
+): Date[] {
+  const parsedDate = parseLocalDateKey(dateKey);
+  const parsedTime = normalizeStartTime(startTime).split(':').map((value) => Number(value));
+  if (!parsedDate || parsedTime.length !== 2) return [];
+
+  const [hour, minute] = parsedTime;
+  const firstSlot = getUtcDateForLocalTime(timezone, parsedDate.year, parsedDate.month, parsedDate.day, hour, minute);
+  const nextLocalDay = new Date(Date.UTC(parsedDate.year, parsedDate.month - 1, parsedDate.day, 0, 0, 0, 0));
+  nextLocalDay.setUTCDate(nextLocalDay.getUTCDate() + 1);
+  const nextDay = getUtcDateForLocalTime(
+    timezone,
+    nextLocalDay.getUTCFullYear(),
+    nextLocalDay.getUTCMonth() + 1,
+    nextLocalDay.getUTCDate(),
+    0,
+    0
+  );
+  const intervalMs = intervalHours * 60 * 60 * 1000;
+  const slots: Date[] = [];
+
+  for (let slotMs = firstSlot.getTime(); slotMs < nextDay.getTime(); slotMs += intervalMs) {
+    slots.push(new Date(slotMs));
+  }
+  return slots;
+}
+
 export function isBackupDueNow(
   destination: BackupDestinationRecord,
   now: Date,
   windowMinutes: number = BACKUP_SCHEDULER_WINDOW_MINUTES
 ): boolean {
   if (!destination.schedule.enabled) return false;
-  const intervalMs = destination.schedule.intervalHours * 60 * 60 * 1000;
   const toleranceMs = Math.max(1, windowMinutes) * 60 * 1000;
   const lastAttemptAt = destination.runtime.lastAttemptAt ? new Date(destination.runtime.lastAttemptAt) : null;
-  if (!lastAttemptAt || !Number.isFinite(lastAttemptAt.getTime())) return true;
-  return now.getTime() - lastAttemptAt.getTime() >= Math.max(0, intervalMs - toleranceMs);
+  const lastAttemptMs = lastAttemptAt && Number.isFinite(lastAttemptAt.getTime())
+    ? lastAttemptAt.getTime()
+    : Number.NEGATIVE_INFINITY;
+  const localDateKey = getBackupLocalDateKey(now, destination.schedule.timezone);
+  const slotStarts = getBackupSlotStartsForLocalDay(
+    localDateKey,
+    destination.schedule.timezone,
+    destination.schedule.startTime,
+    destination.schedule.intervalHours
+  );
+
+  for (const slotStart of slotStarts) {
+    const slotStartMs = slotStart.getTime();
+    if (now.getTime() < slotStartMs || now.getTime() >= slotStartMs + toleranceMs) continue;
+    if (lastAttemptMs >= slotStartMs) return false;
+    return true;
+  }
+  return false;
 }