fix(FN-842): fix new-tab xterm lifecycle for immediate output rendering

- Fix xterm terminal lifecycle so new tabs render output immediately instead of showing blank
- Refactor terminal management: move open/close logic from AgentsView to AgentDetailView per-tab
- Simplify AgentsView and AgentListModal to delegate terminal creation to individual agent tabs
- Update TerminalModal to handle xterm init/resize lifecycle correctly
- Add regression tests for new-tab behavior while modal is already open
- Update README terminal section with new-tab rendering behavior docs

packages/dashboard/app/styles.css

@@ -7201,7 +7201,10 @@ body {
   flex-direction: column;
   align-items: center;
   justify-content: center;
-  height: 100%;
+  position: absolute;
+  inset: 0;
+  z-index: 2;
+  background: #1e1e1e;
   gap: var(--space-lg);
   color: var(--text-muted);
 }