feat: switch from PaperMod to Congo theme

- Warm avocado color scheme, light mode default — friendly for families
- Congo language configs in config/_default/languages.{en,de}.toml
- Renamed extend partials to Congo convention (hyphens)
- Override warnings.html for Hugo 0.159 compatibility (.Author removed)
- All shortcodes and custom partials preserved

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitmodules

@@ -1,3 +1,3 @@
-[submodule "themes/PaperMod"]
-	path = themes/PaperMod
-	url = https://github.com/adityatelange/hugo-PaperMod.git
+[submodule "themes/congo"]
+	path = themes/congo
+	url = https://github.com/jpanther/congo.git