fix: 修复 preload 路径 (.mjs 而不是 .js)

- electron-vite v3 输出 .mjs 文件
- 更新 preload 路径为 index.mjs
- 这是 window.electronAPI undefined 的根本原因

src/renderer/src/stores/connection.ts

@@ -126,6 +126,9 @@ export const useConnectionStore = defineStore('connection', () => {
   async function testConnection(opts: {
     host: string; port: number; auth?: string; username?: string; tls?: boolean
   }): Promise<{ success: boolean; error?: string }> {
+    if (!window.electronAPI) {
+      return { success: false, error: 'Electron API not ready' }
+    }
     const testId = `test_${Date.now()}`
     try {
       const result = await window.electronAPI.redis.connect(testId, {