fix(frontend): remove obsolete @types/react-router-dom

react-router-dom@7 ships its own TypeScript types (./dist/index.d.ts),
making @types/react-router-dom (v5-era) obsolete and a potential source
of type conflicts. Codebase doesn't use any v5-only types
(RouteComponentProps, withRouter, etc.). Frontend build verified.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot

src/frontend/package-lock.json

@@ -20,7 +20,6 @@
     "@radix-ui/react-icons": "^1.3.2",
     "@reduxjs/toolkit": "^2.11.2",
     "@tailwindcss/vite": "^4.2.4",
-    "@types/react-router-dom": "^5.3.3",
     "autoprefixer": "^10.5.0",
     "axios": "^1.15.2",
     "class-variance-authority": "^0.7.1",