fix(meshcore): fix layout height, connection polling, and modal visibility

- Add base flex properties to #meshcoreVisuals so it fills full panel
  height when meshtastic.css hasn't been lazily loaded yet
- Poll /meshcore/status every 2s after Connect click so the UI
  transitions out of "Connecting..." when the backend is ready
- Fix Add Contact and Traceroute modals to use .show class pattern
  (signal-details-modal uses opacity/visibility transitions, not display)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/css/modes/meshcore.css

@@ -20,9 +20,13 @@
     max-width: 100% !important;
 }
 
-/* ── Container overrides ── */
+/* ── Visuals container (base rules duplicated from meshtastic.css — lazy-load safety) ── */
 #meshcoreVisuals {
+    display: flex;
     flex-direction: column;
+    flex: 1;
+    min-height: 0;
+    overflow: hidden;
     padding: 0;
     gap: 0;
 }