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)

pnpm-lock.yaml

@@ -83,9 +83,6 @@ importers:
       zod:
         specifier: ^4.4.3
         version: 4.4.3
-      zod-to-json-schema:
-        specifier: ^3.25.2
-        version: 3.25.2(zod@4.4.3)
     devDependencies:
       '@commitlint/cli':
         specifier: ^21.0.2
@@ -93,9 +90,6 @@ importers:
       '@commitlint/config-conventional':
         specifier: ^21.0.2
         version: 21.0.2
-      '@eslint/eslintrc':
-        specifier: ^3.3.5
-        version: 3.3.5
       '@eslint/js':
         specifier: ^10.0.1
         version: 10.0.1(eslint@9.39.4(jiti@2.7.0))
@@ -117,9 +111,6 @@ importers:
       '@types/react-dom':
         specifier: ^19.2.3
         version: 19.2.3(@types/react@19.2.14)
-      '@types/react-grid-layout':
-        specifier: ^2.1.0
-        version: 2.1.0(react-dom@19.2.5(react@19.2.5))(react@19.2.5)
       '@types/web-push':
         specifier: ^3.6.4
         version: 3.6.4
@@ -1877,10 +1868,6 @@ packages:
     peerDependencies:
       '@types/react': ^19.2.0
 
-  '@types/react-grid-layout@2.1.0':
-    resolution: {integrity: sha512-pHEjVg9ert6BDFHFQ1IEdLUkd2gasJvyti5lV2kE46N/R07ZiaSZpAXeXJAA1MXy/Qby23fZmiuEgZkITxPXug==}
-    deprecated: This is a stub types definition. react-grid-layout provides its own type definitions, so you do not need this installed.
-
   '@types/react@19.2.14':
     resolution: {integrity: sha512-ilcTH/UniCkMdtexkoCN0bI7pMcJDvmQFPvuPvmEaYA/NSfFTAgdUSLAoVjaRJm7+6PvcM+q1zYOwS4wTYMF9w==}
 
@@ -6564,13 +6551,6 @@ snapshots:
     dependencies:
       '@types/react': 19.2.14
 
-  '@types/react-grid-layout@2.1.0(react-dom@19.2.5(react@19.2.5))(react@19.2.5)':
-    dependencies:
-      react-grid-layout: 2.2.3(react-dom@19.2.5(react@19.2.5))(react@19.2.5)
-    transitivePeerDependencies:
-      - react
-      - react-dom
-
   '@types/react@19.2.14':
     dependencies:
       csstype: 3.2.3
@@ -10093,10 +10073,6 @@ snapshots:
     dependencies:
       zod: 3.25.76
 
-  zod-to-json-schema@3.25.2(zod@4.4.3):
-    dependencies:
-      zod: 4.4.3
-
   zod-validation-error@4.0.2(zod@4.4.3):
     dependencies:
       zod: 4.4.3