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.

.env.example

@@ -0,0 +1,31 @@
+# Copy this file to .env.local for local development
+# Then run: npm run dev:local
+
+# Public base URL for your local dev server
+PUBLIC_BASE_URL=http://localhost:4321
+
+# OAuth (Gitea) settings for local development
+# Create an OAuth2 Application in your Gitea with Redirect URI:
+#   http://localhost:4321/api/auth/callback
+# Then paste the resulting Client ID/Secret below
+OAUTH_PROVIDER=gitea
+OAUTH_CLIENT_ID=
+OAUTH_CLIENT_SECRET=
+OAUTH_AUTHORIZE_URL=https://git.bookageek.ch/login/oauth/authorize
+OAUTH_TOKEN_URL=https://git.bookageek.ch/login/oauth/access_token
+OAUTH_USERINFO_URL=https://git.bookageek.ch/api/v1/user
+
+# Optional access control
+# OAUTH_ALLOWED_USERS=user1,user2
+# OAUTH_ALLOWED_ORG=your-org
+
+# Gitea API for committing content changes (service account PAT)
+GITEA_BASE=https://git.bookageek.ch
+GITEA_OWNER=
+GITEA_REPO=
+GITEA_TOKEN=
+GIT_BRANCH=main
+
+# Session and CSRF secrets (use random long strings in .env.local)
+SESSION_SECRET=
+CSRF_SECRET=