Scope PWA manifest to app's own path instead of origin root

Every installed PWA claimed scope "/", so Android link-capturing routed
navigation for any app on the shared origin into whichever PWA had it.

frontend/vite.config.ts

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