fix: BUG-33 CPU threshold, TASK-27 tab icons, TASK-36 iframe errors

- BUG-33: CPU load alert threshold increased from 2x to 4x core count
  (8→16 on 4-core machine) to reduce false alerts during container ops
- TASK-27: Launch buttons for new-tab apps now show external link icon
  (BTCPay, Grafana, PhotoPrism, Portainer, OnlyOffice, etc.)
- TASK-36: Iframe error screen now distinguishes between X-Frame-Options
  blocked vs container not reachable, with appropriate messaging

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

neode-ui/src/views/AppSession.vue

@@ -139,8 +139,11 @@
                   <path stroke-linecap="round" stroke-linejoin="round" stroke-width="1.5" d="M12 15v2m-6 4h12a2 2 0 002-2v-6a2 2 0 00-2-2H6a2 2 0 00-2 2v6a2 2 0 002 2zm10-10V7a4 4 0 00-8 0v4h8z" />
                 </svg>
               </div>
-              <h3 class="text-lg font-semibold text-white mb-2">This site blocks embedded viewing</h3>
-              <p class="text-white/50 text-sm mb-6">{{ appTitle }} sets security headers that prevent iframe embedding.<br>Open it in a new browser tab instead.</p>
+              <h3 class="text-lg font-semibold text-white mb-2">{{ mustOpenNewTab ? 'This app opens in a new tab' : 'App not reachable' }}</h3>
+              <p class="text-white/50 text-sm mb-6">
+                <template v-if="mustOpenNewTab">{{ appTitle }} sets security headers that prevent iframe embedding.<br>Open it in a new browser tab instead.</template>
+                <template v-else>{{ appTitle }} may still be starting up or the container is stopped.<br>Try opening in a new tab or check the app status.</template>
+              </p>
               <button
                 @click="openNewTabAndBack"
                 class="glass-button px-6 py-3 rounded-lg text-sm font-semibold inline-flex items-center gap-2"