refactor(translations): move Help translate into the App language page

Per review, it shouldn't be a top-level Settings entry: the "Help translate"
link now sits at the top of the full-screen App language picker (OptionPicker
gains an optional header slot). Updated README + changelog wording to match.

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

README.md

@@ -118,7 +118,7 @@ language. Translations are managed on a self-hosted **Weblate**:
 
 No coding needed — register on the Weblate server, pick (or request) a language,
 and translate the strings in your browser. You can also reach this link in the
-app under **Settings → Help translate**.
+app from the top of **Settings → App language**.
 
 ## 📜 License