chore: remove dead code and unused dependencies

- Delete src/placeholder.ts (empty stub with no references)
- Remove zod-to-json-schema dep (not imported anywhere in src/)
- Remove @eslint/eslintrc devDep (project uses flat config with @eslint/js)
- Remove @types/react-grid-layout devDep (react-grid-layout v2 ships own types)

package.json

@@ -48,7 +48,6 @@
   "devDependencies": {
     "@commitlint/cli": "^21.0.2",
     "@commitlint/config-conventional": "^21.0.2",
-    "@eslint/eslintrc": "^3.3.5",
     "@eslint/js": "^10.0.1",
     "@playwright/test": "^1.59.1",
     "@release-it/conventional-changelog": "^11.0.1",
@@ -56,7 +55,6 @@
     "@types/node": "^22.9.0",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
-    "@types/react-grid-layout": "^2.1.0",
     "@types/web-push": "^3.6.4",
     "dotenv-cli": "^11.0.0",
     "drizzle-kit": "^0.31.10",
@@ -98,7 +96,6 @@
     "tailwind-merge": "^3.5.0",
     "tw-animate-css": "^1.4.0",
     "web-push": "^3.6.7",
-    "zod": "^4.4.3",
-    "zod-to-json-schema": "^3.25.2"
+    "zod": "^4.4.3"
   }
 }