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>

tsconfig.app.json

@@ -2,7 +2,7 @@
   "extends": "@vue/tsconfig/tsconfig.dom.json",
   "compilerOptions": {
     "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.app.tsbuildinfo",
-    "types": ["vite/client"],
+    "types": ["vite/client", "vitest/globals"],
     "ignoreDeprecations": "6.0",
     "baseUrl": ".",
     "paths": {