fix(ui): implement cached startup pre-flight check in SessionService and propagate errors in StaleBanner

.agents/skills/multi-agent-mux-ui/packages/mam_core/lib/src/services/session_service.dart

@@ -27,7 +27,27 @@ class SessionService {
     this.timeout = const Duration(seconds: 5),
   });
 
+  bool _preflightChecked = false;
+
+  Future<void> _checkPreflight() async {
+    if (_preflightChecked) return;
+
+    final bashResult = await runCommand(['bash', '--version'], timeout: timeout);
+    if (bashResult.rc != 0) {
+      throw StatusFetchException('preflight failed: bash is missing or not executable');
+    }
+
+    final tmuxResult = await runCommand(['tmux', '-V'], timeout: timeout);
+    if (tmuxResult.rc != 0) {
+      throw StatusFetchException('preflight failed: tmux is missing or not executable');
+    }
+
+    _preflightChecked = true;
+  }
+
   Future<SessionsSnapshot> fetchSnapshot() async {
+    await _checkPreflight();
+
     final result = await runCommand(
       ['bash', statusScriptPath, '--json'],
       timeout: timeout,