fix: onboarding 401 redirect, glass card rendering bugs

- rpc-client: don't redirect to /login on 401 during onboarding flow,
  which caused session expired kicks on fresh installs
- style.css: add translateZ(0) + isolation:isolate to glass-card,
  glass-strong, path-option-card to fix Chromium compositor bug where
  backdrop-filter + animated fixed overlays cause black rectangles
- App.vue: pause background animations when tab hidden, force
  compositor layer rebuild on tab return to prevent stale renders

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

neode-ui/src/api/rpc-client.ts

@@ -62,7 +62,9 @@ class RPCClient {
           // Use a single shared timeout to prevent redirect storms when
           // multiple parallel requests all get 401 at once
           if (response.status === 401 && method !== 'auth.login') {
-            if (!RPCClient._sessionExpiredRedirecting) {
+            // Don't redirect during onboarding — those endpoints are unauthenticated
+            const isOnboarding = window.location.pathname.startsWith('/onboarding')
+            if (!isOnboarding && !RPCClient._sessionExpiredRedirecting) {
               RPCClient._sessionExpiredRedirecting = true
               setTimeout(() => {
                 window.location.href = '/login'