feat(FN-3377): compress mobile agent detail header

- Rework AgentDetailView mobile header structure to reduce vertical space while preserving key metadata
- Simplify AgentsView mobile spacing and remove redundant styles tied to legacy header layout
- Update dashboard CSS class coverage tests for new agent detail/mobile header classes
- Expand AgentDetailView and AgentsView tests to validate condensed header rendering and interaction behavior

Fusion-Task-Id: FN-3377

packages/dashboard/app/components/AgentsView.css

@@ -1178,16 +1178,6 @@
     display: none;
   }
 
-  .agents-mobile-back-row {
-    padding: var(--space-md);
-    border-bottom: 1px solid var(--border);
-    background: var(--surface);
-  }
-
-  .agents-mobile-back-btn {
-    min-height: calc(var(--space-lg) * 2 + var(--space-xs));
-  }
-
   .agents-view-content {
     min-height: 0;
     overflow-y: auto;