fix: move ajv to dependencies and auto-install deps in install scripts

`ajv` is required at runtime by the installer (`scripts/lib/install/config.js`)
but was listed under `devDependencies`. This caused `Error: Cannot find module
'ajv'` when running `./install.sh` from a fresh git clone or via `npx`.

- Move `ajv` from devDependencies to dependencies in package.json
- Add auto `npm install` in install.sh when node_modules is missing
- Add matching auto-install in install.ps1 for Windows parity

package.json

@@ -110,11 +110,11 @@
   },
   "dependencies": {
     "@iarna/toml": "^2.2.5",
+    "ajv": "^8.18.0",
     "sql.js": "^1.14.1"
   },
   "devDependencies": {
     "@eslint/js": "^9.39.2",
-    "ajv": "^8.18.0",
     "c8": "^10.1.2",
     "eslint": "^9.39.2",
     "globals": "^17.1.0",