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/app-backup.spec.ts

@@ -28,12 +28,14 @@ test('an admin backs up Gitea data and database in one flow', async ({ page }) =
   // Gitea → select → schedule → target → save
   await gitea.getByRole('button', { name: 'Back up' }).click()
   await expect(page.getByRole('dialog')).toContainText('gitea-shared-storage')
+  // the schedule reads as time, not as cron
+  await expect(page.getByRole('dialog')).toContainText('Every day at 03:00')
   await page.getByLabel('Target', { exact: true }).selectOption({ label: name })
   await page.getByRole('button', { name: 'Create backup plan' }).click()
 
   await expect(page.getByText(/2 backup strategies created/)).toBeVisible()
   await expect(page.getByRole('row', { name: /Gitea – Gitea volume/ })).toBeVisible()
   await expect(page.getByRole('row', { name: /Gitea – PostgreSQL dump/ })).toContainText(
-    '0 0 3 * * *',
+    'Every day at 03:00',
   )
 })