feat: add sign-in flow, routing shell, and searchable book list

Pin @testing-library/jest-dom to ~6.9.1 (6.10.0 is npm-deprecated).

Claude-Session: https://claude.ai/code/session_01M9MLit5Ko3X4s7rzC5Kv7X

package.json

@@ -16,7 +16,7 @@
   "devDependencies": {
     "@tailwindcss/vite": "^4.3.3",
     "@testing-library/dom": "^10.4.2",
-    "@testing-library/jest-dom": "^6.10.0",
+    "@testing-library/jest-dom": "~6.9.1",
     "@testing-library/react": "^16.3.3",
     "@testing-library/user-event": "^14.6.7",
     "@types/node": "^24.10.1",