chore(fdroid): add Summary field to metadata yml

Code review flagged that F-Droid clients use Summary as the canonical
single-line app subtitle on the detail page; without it the slot would
fall back to the package id.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fdroid-metadata/de.jeanlucmakiola.calendula.yml

@@ -1,6 +1,7 @@
 AuthorName: Jean-Luc Makiola
 License: MIT
 Name: Calendula
+Summary: A modern Material 3 Expressive calendar for Android.
 
 Categories:
   - Time