Add Liberapay alongside Ko-fi for donations (#430)

Adds Liberapay as a second donation option next to Ko-fi.

- Settings: "Support development" now opens a full-screen picker with Ko-fi (one-off tips) and Liberapay (recurring donations) instead of linking straight to Ko-fi.
- README: Liberapay badge next to the Ko-fi badge, plus a short Support section.

No issue for this one.

Co-authored-by: Jean-Luc Makiola <business@jeanlucmakiola.de>
Reviewed-on: https://codeberg.org/jlmakiola/calendula/pulls/430

README.md

@@ -23,6 +23,8 @@ It works on top of the system calendar and has no network access.</p>
 <a href="https://apps.obtainium.imranr.dev/redirect?r=obtainium://add/https://codeberg.org/jlmakiola/calendula"><img src="https://github.com/ImranR98/Obtainium/blob/main/assets/graphics/badge_obtainium.png?raw=true" alt="Get it on Obtainium" height="56"></a>
 &nbsp;
 <a href="https://ko-fi.com/jeanlucmakiola"><img src="https://storage.ko-fi.com/cdn/brandasset/v2/support_me_on_kofi_badge_beige.png" alt="Support me on Ko-fi" height="56"></a>
+&nbsp;
+<a href="https://liberapay.com/Jean-Luc_Makiola/donate"><img src="https://liberapay.com/assets/widgets/donate.svg" alt="Donate using Liberapay" height="56"></a>
 </p>
 
 <p>
@@ -203,6 +205,13 @@ anywhere. Your events stay in Android's own calendar store.
 [Privacy policy](https://jeanlucmakiola.de/calendula/privacy), written in
 [`docs/PRIVACY.md`](docs/PRIVACY.md), which is what that page renders.
 
+## Support
+
+Calendula is free and has no ads. If you'd like to support its development,
+you can tip once on [Ko-fi](https://ko-fi.com/jeanlucmakiola) or donate
+regularly on [Liberapay](https://liberapay.com/Jean-Luc_Makiola/donate). Both
+are linked in the app under *Settings → Support development*.
+
 ## License
 
 [MIT](LICENSE), Jean-Luc Makiola, 2026