Add Geografi › Naturgeografi category with 11 curriculum topics, source-linked content, and process demo

[Copilot]

This PR implements the new Geografi › Naturgeografi umbrella area so it is navigable in the subject/category/topic model and usable from folkeskole to gymnasium level.
It adds the requested naturgeografi scope (landskabsdannelse, hydrologi, jordbund, økosystemer) with Danish UI content and explicit source attribution.

• Category + routing surface

  • Added naturgeografi under geografi.categories with 🏔️ label and summary.
  • Registered 11 topic slugs so canonical routes are generated as:
    • /<subject>/<category>/<topic>
    • /<subject>/<category>/<topic>/quiz

• Topic content set (11 topics)

  • New file: content/geografi/naturgeografi.ts
  • Added:
    • Istider og isens landskabsdannelse
    • Gletsjere
    • Floder og vandløb
    • Vandets kredsløb
    • Grundvand
    • Kysttyper og kystdannelse
    • Ørkener og ørkendannelse
    • Skove og biomer
    • Jordbund og jordtyper
    • Forvitring og erosion
    • Søer og vådområder
  • Each topic includes: Danish summary, concept sections, quiz items, and source-oriented sections.
  • Includes requested instructional elements across topics: case studies, cross-section/map references, and process-focused explanations.

• Animation/demo integration

  • Added components/NaturgeografiProcesserDemo.tsx.
  • Wired demo into existing demoKey mapping in topic page rendering.
  • Scoped usage to the core glacial-landscape topic and improved accessibility labeling in the SVG diagram.

• Source attribution quality

  • Added/expanded explicit source URLs in content sections.
  • Preserved figure-level source metadata through existing image.sourcePage pattern for rendered credits/links.

// content/geografi/index.ts
{
  slug: "naturgeografi",
  title: "Naturgeografi",
  emoji: "🏔️",
  topics: [
    istiderOgIsensLandskabsdannelse,
    gletsjere,
    floderOgVandloeb,
    vandetsKredsloeb,
    grundvand,
    kysttyperOgKystdannelse,
    oerkenerOgOerkendannelse,
    skoveOgBiomer,
    jordbundOgJordtyper,
    forvitringOgErosion,
    soeerOgVaadomraader,
  ],
}

[rastermanden]

Superseded by #70, which has been merged into main. The single-file content/geografi/naturgeografi.ts and the NaturgeografiProcesserDemo component were preserved verbatim (with one fix: added missing "use client" so the demo can cross the server/client boundary); demoKey was converted to demoComponent. Registered in a new naturgeografi category against the per-category structure from #61. Closing.

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