feat: add Vitest testing framework, fix proxy auth stripping and 401 handling

- Set up Vitest with jsdom for client tests, node for server tests
- Add tests for auth service, proxy handler, API client, and profiles store
- Strip Authorization header in proxy to prevent web-ui token leaking to gateway
- Distinguish local BFF vs proxied gateway 401s to avoid false logouts
- Remove unused hero.png asset

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

vitest.config.ts

@@ -0,0 +1,14 @@
+import { defineConfig } from 'vitest/config'
+import { resolve } from 'path'
+
+export default defineConfig({
+  resolve: {
+    alias: {
+      '@': resolve(__dirname, 'packages/client/src'),
+    },
+  },
+  test: {
+    include: ['tests/**/*.test.ts'],
+    setupFiles: ['tests/setup.ts'],
+  },
+})