Add electron-builder packaging configuration

- Install electron-builder as dev dependency
- Configure build targets for macOS (dmg, zip), Windows (nsis, zip), and Linux (AppImage, deb)
- Add npm scripts for building unpacked app (dist:dir) and macOS packages (dist:mac)
- Set base path to "./" in production builds for proper asset loading in packaged app
- Fix index.html asset paths to use relative paths (./ prefix)
- Update README to document packaging commands and note unsigned/unnotarized status

vite.config.ts

@@ -5,8 +5,9 @@ import react from "@vitejs/plugin-react";
 const host = process.env.TAURI_DEV_HOST;
 
 // https://vite.dev/config/
-export default defineConfig(async () => ({
+export default defineConfig(async ({ command }) => ({
   plugins: [react()],
+  base: command === "build" ? "./" : "/",
 
   // Vite options tailored for Tauri development and only applied in `tauri dev` or `tauri build`
   //