Fix electron-builder: remove type:module, use vite.config.mts instead

The "type": "module" in package.json caused tsup to emit .cjs files instead
of .js, breaking the electron-builder entry point check. Using .mts extension
for vite config achieves ESM for Vite without affecting the rest of the package.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vite.config.mts

@@ -0,0 +1,14 @@
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+import path from "node:path";
+
+export default defineConfig({
+  plugins: [react()],
+  base: "./",
+  root: path.resolve(__dirname, "src/renderer"),
+  publicDir: path.resolve(__dirname, "assets"),
+  build: {
+    outDir: path.resolve(__dirname, "build/renderer"),
+    emptyOutDir: true
+  }
+});