fix: add build target es2015 for Raspberry Pi Chromium compatibility

Older Chromium on ARM64 doesn't support strict mode arguments.callee
in modern ES module bundles. Downgrading build target to es2015.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

frontend/vite.config.ts

@@ -14,6 +14,7 @@ export default defineConfig(({ command }) => ({
     }
   },
   build: {
+    target: "es2015",
     chunkSizeWarningLimit: 900,
     rollupOptions: {
       output: {