feat(ZEE-29): implement design system, dark mode, and JSON content management

- Add comprehensive design system with CSS variables, animations, and utility classes
- Implement dark mode with ThemeProvider (system preference + manual toggle)
- Create JSON-based content management system in src/content/
- Update all pages to use structured JSON content
- Add ThemeProvider component with theme toggle button
- Update Header with glass effect and animated mobile menu
- Update Footer with dark mode support
- Update documentation (README.md, CLAUDE.md, knowledge.md, proto/README.md)
- Sync with Linear (ZEE-29 completed)

proto/src/content/pages/home.json

@@ -0,0 +1,10 @@
+{
+  "cta": {
+    "title": "Kapcsolatfelvétel az első lépés",
+    "subtitle": "Mutassuk meg, hogyan segíthetünk Önnek megvalósítani céljait!",
+    "button": "Kapcsolatfelvétel"
+  },
+  "serviceFeatures": {
+    "title": "Szolgáltatás jellemzők:"
+  }
+}