fix: build frontend and fix tsc errors blocking the build

vite.config.ts used defineConfig from 'vite' which doesn't accept the
'test' key — switch to 'vitest/config' which extends it. Also type the
onChange test helper as (knob: KnobConfig) => void instead of the overly
broad ReturnType<typeof vi.fn>. Run npm run build to ship the compiled
static assets.

ui/src/__tests__/KnobCard.test.tsx

@@ -39,12 +39,12 @@ interface RenderOptions {
   runningApps?: string[];
   sinks?:       AudioDevice[];
   sources?:     AudioDevice[];
-  onChange?:    ReturnType<typeof vi.fn>;
+  onChange?:    (knob: KnobConfig) => void;
 }
 
 function renderCard(
   overrides: Partial<KnobConfig> = {},
-  { runningApps, sinks, sources, onChange = vi.fn() }: RenderOptions = {},
+  { runningApps, sinks, sources, onChange = vi.fn() as (knob: KnobConfig) => void }: RenderOptions = {},
 ) {
   const knob: KnobConfig = {
     action: 'app_volume',

ui/vite.config.ts

@@ -1,4 +1,5 @@
-import { defineConfig } from 'vite'
+/// <reference types="vitest" />
+import { defineConfig } from 'vitest/config'
 import react from '@vitejs/plugin-react'
 
 // https://vite.dev/config/