Fix all-workers-offline silent success bug, noisy logging, UX polish

server.js:
- Fix bug: when all targeted workers disconnect before step runs, results[] was empty
  and results.every() returned true vacuously (silent false success). Now tracks sentCount
  and fails with 'no_workers' log if nothing was actually dispatched
- Remove per-message console.log on every WebSocket message (high noise)
- Only log a warning for failed commands (not every success)

index.html:
- loadSchedules() catch now shows error message in scheduleList (was silent)
- abortExecution() shows server's error message from JSON body instead of generic string
  (e.g. "Execution is not running" instead of "Failed to abort execution")

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

public/index.html

@@ -1376,6 +1376,7 @@
                 document.getElementById('scheduleList').innerHTML = html;
             } catch (error) {
                 console.error('Error loading schedules:', error);
+                document.getElementById('scheduleList').innerHTML = '<div class="empty" style="color:var(--terminal-red);">⚠ Failed to load schedules</div>';
             }
         }
 
@@ -2369,7 +2370,8 @@
                     closeModal('viewExecutionModal');
                     refreshData();
                 } else {
-                    alert('Failed to abort execution');
+                    const err = await response.json().catch(() => ({}));
+                    alert(err.error || 'Failed to abort execution');
                 }
             } catch (error) {
                 console.error('Error aborting execution:', error);