fix(meshcore): surface backend error messages and extend polling window

- Store last status message on MeshcoreClient so error details survive
  beyond the SSE event (which isn't active during connecting state)
- Status endpoint now returns message field so the frontend can show
  the real reason (e.g. 'Connection failed after retries: ...')
- Extend JS polling from 30s to 90s to outlast the backend's 65s
  retry sequence (5+15+45s delays) before declaring timeout

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

static/js/modes/meshcore.js

@@ -97,7 +97,8 @@ const MeshCore = (function () {
 
     function _pollUntilConnected(attempts) {
         if (_connected) return;
-        if (attempts > 15) {
+        if (attempts > 45) {
+            // Backend retry window (5+15+45s) has elapsed — give up
             _updateStatusUI('error', 'Connection timed out');
             return;
         }