notifications: write gsettings only when the theme changed

From the final review. Every save ran gsettings set, so saving a timeout
on a first run, with no soundTheme in the file, silently reset a system
theme set elsewhere to freedesktop; the page now writes it only when the
draft's theme differs from the file's, and flags any other mismatch as
before. An exit code outside 0-3, a save killed mid-run, now reads as an
error rather than a partial success. The player's comment no longer
claims the balloon's expiry check, which it does not need.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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