fix: use :global(html.dark .selector) for scoped dark mode rules

Vue scoped styles append [data-v-xxx] to child selectors, so
`:global(html.dark) .foo` compiled to `.foo[data-v-xxx]` which never
matched. Wrapping the full rule in :global() prevents Vue from adding
the scope attribute.

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

frontend/src/App.vue

@@ -308,7 +308,7 @@ body { font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-
   overflow: hidden;
 }
 
-html.dark .sidebar { background: #020617; }
+:global(html.dark .sidebar) { background: #020617; }
 
 .logo {
   display: flex;

frontend/src/components/LoginPage.vue

@@ -153,7 +153,7 @@ async function submit() {
   padding: 8px 12px;
 }
 
-:global(html.dark) .login-error {
+:global(html.dark .login-error) {
   background: #2A1515;
   border-color: #3D1F1F;
 }