Pick and show schedules as plain time instead of cron

Schedules were entered and displayed as six field cron expressions. A
picker now offers a frequency, a weekday or day of the month and a time,
and turns that into the expression the backend stores; tables and the
dashboard show wording like "Every day at 03:00". An expression the
picker cannot express stays editable as cron.

This also fixes a broken preset: the backend's cron dialect counts Sunday
as 1 and rejects 0, so the "every Sunday" choice in the application
backup form produced an invalid expression. The picker emits weekday
names, and a test pins the dialect down.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/e2e/backups.spec.ts

@@ -26,7 +26,7 @@ test('admin creates a target and a strategy, runs it and sees the backup', async
   await page.getByLabel('Source type', { exact: true }).selectOption('postgres_dump')
   await page.getByLabel('Namespace', { exact: true }).fill('gitea')
   await page.getByLabel('Pod', { exact: true }).fill('gitea-postgresql-0')
-  await page.getByLabel('Schedule', { exact: true }).fill('0 0 2 * * *')
+  await page.getByLabel('At', { exact: true }).fill('02:00')
   await page.getByLabel('Target', { exact: true }).selectOption({ label: name })
   await page.getByLabel('Keep last', { exact: true }).fill('3')
   await page.getByRole('button', { name: 'Save strategy' }).click()
@@ -36,4 +36,5 @@ test('admin creates a target and a strategy, runs it and sees the backup', async
   await expect(row).toContainText(/gitea-database_.*\.sql\.gz/, { timeout: 20_000 })
   await row.getByRole('button', { name: 'History' }).click()
   await expect(page.getByTestId('backup-records')).toContainText('sql.gz')
+  await expect(row).toContainText('Every day at 02:00')
 })