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>

layouts/partials/extend-head.html

@@ -0,0 +1,30 @@
+{{/* Structured data for SEO + AI search */}}
+{{ partial "structured-data.html" . }}
+
+{{/* Hreflang tags for multilingual SEO */}}
+{{ if .IsTranslated }}
+<link rel="alternate" hreflang="{{ .Language.Lang }}" href="{{ .Permalink }}" />
+{{ range .Translations }}
+<link rel="alternate" hreflang="{{ .Language.Lang }}" href="{{ .Permalink }}" />
+{{ end }}
+{{ end }}
+
+{{/* Canonical URL */}}
+<link rel="canonical" href="{{ .Permalink }}" />
+
+{{/* AI Search Engine Optimization */}}
+<meta name="robots" content="index, follow, max-snippet:-1, max-image-preview:large" />
+<meta name="googlebot" content="index, follow" />
+
+{{ if .IsPage }}
+<meta name="topic" content="{{ .Title }}" />
+<meta name="subject" content="{{ .Description | default .Summary | plainify | truncate 160 }}" />
+{{ with .Params.tags }}<meta name="keywords" content="{{ delimit . ", " }}" />{{ end }}
+<meta name="article:published_time" content="{{ .Date.Format "2006-01-02" }}" />
+<meta name="article:modified_time" content="{{ .Lastmod.Format "2006-01-02" }}" />
+<meta name="article:section" content="{{ .Section }}" />
+<meta name="article:author" content="{{ .Params.author | default "Agiliton" }}" />
+{{ end }}
+
+{{/* LLMs.txt discovery */}}
+<link rel="alternate" type="text/plain" href="/llms.txt" title="LLM-readable site description" />