Remediate llm-fe npm audit critical/high vulnerabilities (#69)

## Summary
- Closes #53
- `npm audit` in `llm-fe`: **103 → 4** (0 critical, 0 high; 4 moderate remain)
- Applied `npm audit fix`, bumped direct floors (`axios`, `react-router-dom`), pinned `styled-components@6.1.15` for CRA typecheck stability
- Added npm `overrides` for vulnerable transitive leaves (`tar`, `postcss`, `svgo`, `sharp`, `ws`, `flatted`, etc.)
- Vendored CRA-compatible `brace-expansion@5.0.8` shim (v1 default-export API + GHSA-mh99 DoS limits)

## Remaining (documented)
- **moderate:** `react-router`/`react-router-dom` — needs v7 (breaking)
- **moderate:** `webpack-dev-server` via `react-scripts` — CRA incompatible with patched WDS 5.2.6+; needs CRA → Vite (or similar) follow-up
- Deprecation warnings from CRA/`eslint@8`/old `glob` tooling still present until toolchain migrate

## Test plan
- [x] `npm ci` in `llm-fe`
- [x] `npm audit` → 0 critical / 0 high
- [x] `npm run build`
- [x] `npm run test:ci` → 25 suites / 112 tests passedReviewed-on: #69

llm-fe/vendor/brace-expansion-compat/package.json

@@ -0,0 +1,7 @@
+{
+  "name": "brace-expansion",
+  "version": "5.0.8",
+  "description": "v1-compatible brace-expansion@5.0.8 shim (GHSA-mh99-v99m-4gvg)",
+  "main": "index.js",
+  "license": "MIT"
+}