Fix linter errors in backend & main process

- Add missing return type annotations to TypeScript functions
- Fix prettier formatting issues
- Fix unused variable errors
- Configure ESLint to not require return types for .js files
- Add JSDoc documentation to bin/cli.js

Files changed:
- bin/cli.js: Add JSDoc documentation for forwardSignal function
- electron.vite.config.ts: Add return types to copyResources plugin
- eslint.config.mjs: Fix prettier formatting and add .js file exception
  for explicit-function-return-type rule
- src/main/: Add return types across agent, ipc, checkpointer, services, storage
- src/preload/: Fix prettier formatting in index.ts and index.d.ts

All linter errors fixed in main process and backend code.

bin/cli.js

@@ -42,7 +42,11 @@ const child = spawn(electron, [mainPath, ...args], {
   stdio: 'inherit'
 })
 
-// Forward signals to child process
+/**
+ * Forward signals to child process
+ * @param {NodeJS.Signals} signal - The signal to forward
+ * @returns {void}
+ */
 function forwardSignal(signal) {
   if (child.pid) {
     process.kill(child.pid, signal)