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>

config/_default/params.toml

@@ -0,0 +1,18 @@
+colorScheme = "avocado"
+defaultAppearance = "light"
+autoSwitchAppearance = false
+
+[homepage]
+  layout = "page"
+  showRecent = true
+  recentLimit = 5
+
+[article]
+  showDate = true
+  showReadingTime = true
+  showAuthor = true
+  showBreadcrumbs = true
+  showTableOfContents = true
+
+[list]
+  showBreadcrumbs = true