Group the Sources tab by kind and pick a kind when adding a feed (#4)

The Sources tab shows News, Learning and Fun sections, each split by
language; kinds or languages without Sources get no header. The add-feed
dialog has a kind picker that defaults to News.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -34,7 +34,8 @@ or write a daily briefing. It's meant to replace the n8n → Discord workflow.
   valid for a day) and reuses the cookie for reader view and the in-app page.
   With it off, the app shows the teaser and an "Open website anyway" option.
 - **Sources**: 23 curated feeds (13 German, 10 English), toggle each one, or add any
-  RSS/Atom feed (it's validated before being added).
+  RSS/Atom feed as a News, Learning or Fun source (it's validated before being
+  added). The tab groups Sources by kind, then by language.
 - **Dark mode**: System / Light / Dark in Settings (applies instantly). The
   original-page view follows it too: when the phone is dark, sites use their own
   dark design or Android darkens them; when only the app is dark, the app inverts