refactor(ipc): harden channels and unify IPC contracts

tests/unit/preload-surface.test.ts

@@ -0,0 +1,11 @@
+import { describe, expect, it } from 'vitest'
+import fs from 'fs'
+import path from 'path'
+
+describe('Preload API Surface', () => {
+  it('does not expose ipcRenderer in window typings', () => {
+    const dtsPath = path.resolve(process.cwd(), 'src/preload/index.d.ts')
+    const content = fs.readFileSync(dtsPath, 'utf-8')
+    expect(content.includes('ipcRenderer:')).toBe(false)
+  })
+})