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>

core/archipelago/src/monitoring/mod.rs

@@ -108,9 +108,9 @@ impl AlertRule {
             },
             AlertRule {
                 kind: AlertRuleKind::CpuLoad,
-                threshold: 2.0,
+                threshold: 4.0,
                 enabled: true,
-                description: "CPU load exceeds 2x core count for 5 minutes".to_string(),
+                description: "CPU load exceeds 4x core count for 5 minutes".to_string(),
             },
             AlertRule {
                 kind: AlertRuleKind::ContainerCrash,