v3: fix: keep enable() working when usage.json is corrupt

JSON.parse errors have no matches(), so the read-usage error handler threw
and enable() stopped before starting the poll timer. Check for GLib.Error
before matching, and skip restyling/re-sizing the label when nothing changed.

v2: ci: make release creation idempotent

Update the GitHub release workflow to handle re-runs and re-tags gracefully:
- Check if release already exists before creating
- Upload asset with --clobber flag for existing releases
- Extract ZIP filename to variable for clarity