feat: implement GitHub OIDC authentication

- Add GitHub OAuth 2.0 authentication flow
- Create authentication routes: /auth/login, /auth/callback, /auth/logout
- Implement OAuth server utilities with proper session management
- Add authentication hooks and client-side state management
- Update header with login/logout controls and user menu
- Create user profile page with GitHub integration
- Add environment variables for GitHub OAuth configuration
- Include comprehensive documentation for setup and usage
- Enhance profile store with authentication state
- Add authentication status API endpoint

Closes: GitHub authentication implementation

.env.example

@@ -1,5 +1,24 @@
 # Rename this file to .env once you have filled in the below environment variables!
 
+# GitHub OAuth Authentication Configuration
+# ---------------------------------------
+# To set up GitHub OAuth:
+# 1. Go to https://github.com/settings/developers
+# 2. Click "New OAuth App"
+# 3. Fill in the application details:
+#    - Application name: Buildify (or your preferred name)
+#    - Homepage URL: http://localhost:5173 (or your deployment URL)
+#    - Authorization callback URL: http://localhost:5173/auth/callback
+# 4. Click "Register application"
+# 5. Copy the Client ID and generate a new Client Secret
+# 6. Paste them below
+GITHUB_CLIENT_ID=
+GITHUB_CLIENT_SECRET=
+
+# Session Secret - Used to encrypt session cookies
+# Generate a random string (e.g., using `openssl rand -hex 32` in terminal)
+SESSION_SECRET=
+
 # Get your GROQ API Key here -
 # https://console.groq.com/keys
 # You only need this environment variable set if you want to use Groq models