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)

src/placeholder.ts

@@ -1 +0,0 @@
-export {};