chore(frontend): gitignore tsbuildinfo + shadcn to devDeps

Untrack tsconfig.tsbuildinfo (TS build artifact) and add *.tsbuildinfo
to .gitignore. Move shadcn from dependencies to devDependencies (CLI
generator, not runtime). npm test passes; npm run build clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -20,7 +20,6 @@
     "next-themes": "^0.4.6",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
-    "shadcn": "^4.12.0",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.6.0",
     "tw-animate-css": "^1.4.0",
@@ -37,6 +36,7 @@
     "jsdom": "^25.0.0",
     "msw": "^2.4.0",
     "postcss": "^8.4.0",
+    "shadcn": "^4.12.0",
     "tailwindcss": "^3.4.0",
     "typescript": "^5.6.0",
     "vite": "^6.0.0",