Implement OAuth authentication and admin panel

- Introduced OAuth-based login flow with session management and CSRF protection.
- Added admin panel for managing events and gallery content with real-time editing functionality.
- Integrated Gitea API for saving files and updating repository content.
- Updated `.env.example` to include OAuth and Gitea-related configurations.
- Added example event and gallery JSON files for demonstration.

src/content/gallery.json

@@ -0,0 +1,12 @@
+[
+  { "src": "/images/Logo.png", "alt": "Erstes Bild" },
+  { "src": "/images/Logo.png", "alt": "Zweites Bild" },
+  { "src": "/images/Logo.png", "alt": "Drittes Bild" },
+  { "src": "/images/Logo.png", "alt": "Viertes Bild" },
+  { "src": "/images/Logo.png", "alt": "Fünftes Bild" },
+  { "src": "/images/Logo.png", "alt": "Sechstes Bild" },
+  { "src": "/images/Logo.png", "alt": "Siebtes Bild" },
+  { "src": "/images/Logo.png", "alt": "Achtes Bild" },
+  { "src": "/images/Logo.png", "alt": "Neuntes Bild" },
+  { "src": "/images/Logo.png", "alt": "Zehntes Bild" }
+]