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>

app/src/main/res/values-de/strings.xml

@@ -315,6 +315,7 @@
     <string name="settings_license_value">MIT</string>
     <string name="settings_about_author">von Jean-Luc Makiola</string>
     <string name="settings_about_source">Quellcode</string>
+    <string name="settings_about_support">Entwicklung unterstützen</string>
     <string name="settings_about_version">Version %1$s</string>
     <string name="settings_about_logo_desc">Calendula-App-Symbol</string>
     <string name="settings_report_problem">Problem melden</string>