Add kids news channel; document per-source language filter

- New kids channel (/kids) with theme color, surfaced in the web UI
  and routed to by BBC Newsround, DOGO News, Science News Explores,
  and NASA for Students in the example config.
- README and config.example.toml now mention the optional
  per-source `language` filter added in the previous commit.

internal/web/server.go

@@ -38,6 +38,7 @@ var channels = []Channel{
 	{Slug: "music", Title: "Music", Theme: "music", Emoji: "🎵", Blurb: "Records, scenes, and the artists shaping the sound."},
 	{Slug: "anime", Title: "Anime", Theme: "anime", Emoji: "🌸", Blurb: "Series, studios, and the wider world of anime and manga."},
 	{Slug: "foss", Title: "FOSS", Theme: "foss", Emoji: "🐧", Blurb: "Kernel, distros, and the wider free and open source software world."},
+	{Slug: "kids", Title: "Kids", Theme: "kids", Emoji: "🧒", Blurb: "World news written for younger readers — short, clear, and curious."},
 }
 
 // SourceInfo is the trimmed view of a configured feed for the settings panel.