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>

backend/package.json

@@ -13,6 +13,11 @@
     "ws": "^8.14.2",
     "cors": "^2.8.5",
     "uuid": "^9.0.0",
-    "multer": "^1.4.5-lts.1"
+    "multer": "^1.4.5-lts.1",
+    "express-session": "^1.18.0",
+    "connect-redis": "^7.1.0",
+    "redis": "^4.6.0",
+    "openid-client": "^5.6.0",
+    "cookie-parser": "^1.4.6"
   }
 }