feat: Add OIDC authentication with Authentik integration

- Add OIDC login flow with Authentik provider
- Implement session-based auth with Redis store
- Add avatar display from OIDC claims
- Fix input field performance with react-textarea-autosize
- Stabilize callbacks to prevent unnecessary re-renders
- Fix history loading to skip empty session files
- Add 2-row default height for input textarea

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,3 @@
+# Netbird Setup Key from https://gw.sneakercloud.de
+# Create a new setup key for "claude-webui" with appropriate permissions
+NETBIRD_SETUP_KEY=your-setup-key-here