Options: hide empty startup notes and balance the update label

[emaspa]

Summary

Hide the startup note when it is empty, using the same visibility guard as the submixer note. When present, indent it under the checkbox label. This removes the blank row below Start the daemon in the Options window.

Also put (at most once per day) on its own line in the update-check checkbox, matching #76 for main.

This PR targets packaging/flatpak, not main. No other Flatpak work or insert-button changes are part of this diff.

Verification

• Options UI Release build with warnings treated as errors: 0 warnings, 0 errors.
• Native-host-enabled solution build with both Options changes and the already-merged insert-button fix: 0 warnings, 0 errors.
• Layout probe against the applied markup shows uniform 38px checkbox spacing instead of the extra 20px blank row, and the update text fits on two balanced rows.
• The user confirmed the fixes in the running app after UI-only deployment. The daemon and audio were not restarted.

No version bump or setting behaviour changes.