[TS] Improve various types / remove assertions (#4148)

vite.electron.config.mts

@@ -1,6 +1,5 @@
 import { Plugin, defineConfig } from 'vite'
 import { mergeConfig } from 'vite'
-import type { UserConfig } from 'vitest/config'
 
 import baseConfig from './vite.config.mts'
 
@@ -83,7 +82,7 @@ const mockElectronAPI: Plugin = {
 }
 
 export default mergeConfig(
-  baseConfig as unknown as UserConfig,
+  baseConfig,
   defineConfig({
     plugins: [mockElectronAPI]
   })