Merge feature/source-kinds into ticket/3-fun-section (#3)

Keeps the Learning catalog from #6 and puts the Fun group after it, in the
catalog and the README. Adds the end-to-end check #4 left for Fun: a custom
Fun feed added on the Sources tab shows up in the Learn tab's Fun section,
not in Today.

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

README.md

@@ -8,6 +8,9 @@ or write a daily briefing. It's meant to replace the n8n → Discord workflow.
 
 - **Today**: all articles from the enabled sources in the last 12/24/48 h, newest
   first. Filter by language or source, pull to refresh.
+- **Learn**: articles from the enabled Learning sources (engineering blogs,
+  system design explainers, AI research write-ups) from the last 14 days, newest
+  first. Learning sources aren't part of the Briefing.
 - **Reading in the app**: tapping an article opens a clean **reader view** (text,
   headings, lists, quotes, images) without ads or cookie banners. The globe icon
   switches to the **original page** in an in-app WebView, for paywalls or sites
@@ -34,7 +37,8 @@ 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
@@ -180,6 +184,36 @@ page. Hacker News and Lobsters link to articles all over the web, so reader view
 depends on the linked site. Techmeme links to its own summary page, which only
 has the teaser; the original page links on to the full story.
 
+## Learning sources
+
+Evergreen reading for building skills: research, engineering depth and system
+design rather than product news. The Learn tab shows their posts from the last
+14 days. All are English; ★ marks the ones enabled by default.
+
+| | Source | Feed | Focus |
+|---|---|---|---|
+| 🇬🇧 ★ | ByteByteGo | `blog.bytebytego.com/feed` | System design explained with diagrams |
+| 🇬🇧 ★ | System Design One | `newsletter.systemdesign.one/feed` | System design case studies of real systems |
+| 🇬🇧 ★ | The Pragmatic Engineer (blog) | `blog.pragmaticengineer.com/rss/` | How big tech and startups build software (free posts) |
+| 🇬🇧 ★ | Martin Fowler | `martinfowler.com/feed.atom` | Software architecture, refactoring, delivery |
+| 🇬🇧 ★ | Julia Evans | `jvns.ca/atom.xml` | Systems, networking and Unix tools, explained plainly |
+| 🇬🇧 ★ | Dan Luu | `danluu.com/atom.xml` | Long, data-driven essays on computers and engineering |
+| 🇬🇧 ★ | The Cloudflare Blog | `blog.cloudflare.com/rss/` | Networking, security, incident write-ups |
+| 🇬🇧 ★ | Netflix Technology Blog | `netflixtechblog.com/feed` | Distributed systems, data, streaming at scale |
+| 🇬🇧 ★ | Engineering at Meta | `engineering.fb.com/feed/` | Infrastructure, AI, production engineering |
+| 🇬🇧 ★ | GitHub Blog: Engineering | `github.blog/engineering/feed/` | How GitHub builds and runs GitHub |
+| 🇬🇧 ★ | Latent Space | `www.latent.space/feed` | AI engineering: interviews and deep dives |
+| 🇬🇧 ★ | Import AI | `importai.substack.com/feed` | Weekly AI research digest by Jack Clark |
+| 🇬🇧 ★ | Interconnects | `www.interconnects.ai/feed` | Language model training and research |
+| 🇬🇧 ★ | Ahead of AI | `magazine.sebastianraschka.com/feed` | LLM research explained by Sebastian Raschka |
+| 🇬🇧 ★ | Lilian Weng | `lilianweng.github.io/index.xml` | Thorough surveys of machine learning topics |
+| 🇬🇧 | InfoQ | `feed.infoq.com/` | Architecture and engineering talks, high volume |
+
+Left out on purpose: The Pragmatic Engineer newsletter (mostly paid), One Useful
+Thing (applied AI), vendor blogs (OpenAI, DeepMind, Hugging Face) and Chip Huyen
+(inactive since January 2025). Some feeds are long (Dan Luu ~130 posts, Lilian
+Weng ~50); the 14-day window keeps the list short.
+
 ## Fun sources
 
 Tech humour for the Fun section of the Learn tab, which reaches back 3 days. Fun