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>

frontend/package.json

@@ -14,6 +14,7 @@
     "react-dom": "^18.2.0",
     "react-markdown": "^10.1.0",
     "react-syntax-highlighter": "^16.1.0",
+    "react-textarea-autosize": "^8.5.9",
     "remark-gfm": "^4.0.1"
   },
   "devDependencies": {