fix(fdroid): correct metadata format to fastlane convention + add icon.png

Inspection of the local Hetzner-synced F-Droid repo after v0.1.0
revealed that fdroidserver only partially picked up Calendula's
metadata: summary was sourced from the YAML fallback (en-US only),
description appeared only for the "de" locale (not de-DE), and no
icon was shown anywhere. Root cause: we wrote Google Play conventions
(short_description.txt, full_description.txt, bare locale code "de")
where fdroidserver expects the fastlane format that the sibling
HouseHoldKeaper repo already uses successfully.

Changes:
- de/ -> de-DE/ (BCP-47 with region matches HHK and is more reliably
  parsed by fdroidserver)
- short_description.txt -> summary.txt
- full_description.txt -> description.txt
- Add icon.png (512x512) per locale, composed from the adaptive icon's
  foreground path + slate background (rendered via rsvg-convert).
  Required because XML-only adaptive icons in the APK aren't
  auto-rasterized by fdroidserver.

Verified locally against the previously-broken index by composing the
new icon and renaming the files in-tree.

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

fdroid-metadata/de.jeanlucmakiola.calendula/de-DE/description.txt

@@ -0,0 +1,12 @@
+Calendula ist eine moderne, quelloffene Kalender-App für Android. Sie liest
+direkt aus dem System-Kalender-Provider — jede Quelle, die mit deinem Gerät
+synchronisiert ist (Nextcloud über DAVx5, Google, lokal, WebCal-Subscriptions)
+erscheint automatisch.
+
+Der Unterschied liegt im Design: echtes Material 3 Expressive durchgehend,
+mit Dynamic Color, expressiven Animationen und neuen Shape-Sprachen.
+
+V1 ist read-only. Erstellen, Bearbeiten und Löschen von Events kommt mit V2.
+
+Datenschutz: keinerlei Telemetrie, kein Tracking, kein Netzwerkzugriff — deine
+Daten bleiben auf dem Gerät.