Fix PWA manifest scope to / per install-architecture doc

Scope was set to the app's own base path, which breaks the installed
PWA out of standalone mode into a regular browser tab on any
same-origin navigation outside that path (e.g. the auth redirect).
scope: "/" keeps navigation inside the installed app's window.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -12,7 +12,7 @@ export default defineConfig({
         name: 'HK Planner',
         short_name: 'HK Planner',
         start_url: '/hk-planner/',
-        scope: '/hk-planner/',
+        scope: '/',
         display: 'standalone',
         theme_color: '#2d6a4f',
         background_color: '#2d6a4f',