feat(about): add Ko-fi support link

Add an optional way for users to support development:

- About card: tonal "Support development" button opening ko-fi.com
  via ACTION_VIEW (no INTERNET permission), EN + DE strings
- F-Droid metadata: Donate field in self-hosted and official drafts
- README: short Support section with the Ko-fi link

No perks/rewards attached — kept as a plain donation link.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/fdroid-official/de.jeanlucmakiola.calendula.yml

@@ -21,6 +21,7 @@ AuthorName: Jean-Luc Makiola
 SourceCode: https://gitea.jeanlucmakiola.de/makiolaj/calendula
 IssueTracker: https://gitea.jeanlucmakiola.de/makiolaj/calendula/issues
 Changelog: https://gitea.jeanlucmakiola.de/makiolaj/calendula/src/branch/main/CHANGELOG.md
+Donate: https://ko-fi.com/jeanlucmakiola
 
 AutoName: Calendula