Clarify contacts and calendar backup preference subtitle

[xhon-pelushi]

Summary

• Change prefs_daily_backup_summary so it no longer implies daily backups are already enabled.
• Use the wording suggested on the issue: configure automatic or one-time backups.

Fixes #4020

Test plan

• Open Settings → Contacts & calendar backup with daily backup unset and confirm the subtitle does not claim daily backup is active.
• Confirm the preference still opens the backup setup screen as before.
• String-only change in values/strings.xml (translations update via the usual process).

[github-actions]

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