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>

fastlane/metadata/android/en-US/changelogs/21100.txt

@@ -30,9 +30,10 @@
 - Field icons in the event-form settings. Each optional-field toggle (Settings →
   Event form) now shows the same icon the field uses in the new-event form, so
   the list is easier to scan.
-- Help translate Calendula. A new **Settings → Help translate** link opens the
-  project's Weblate, where you can add or improve a language in your browser — no
-  coding needed. Contributions in any language are welcome.
+- Help translate Calendula. A **Help translate** link at the top of **Settings →
+  App language** opens the project's Weblate, where you can add or improve a
+  language in your browser — no coding needed. Contributions in any language are
+  welcome.
 
 ### Changed
 - Tidied **Settings → Appearance** into clearer groups (theme & colour, calendar,