Fix double-submit bug in task creation form

- Add isSubmitting flag to prevent duplicate submissions
- Disable submit button and show "Creating..." text while processing
- Re-enable button after completion or error

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/aggregator/web/index.html

@@ -589,9 +589,19 @@
       }
     };
 
+    let isSubmitting = false;
     document.getElementById('new-task-form').onsubmit = async (e) => {
       e.preventDefault();
 
+      // Prevent double submission
+      if (isSubmitting) return;
+      isSubmitting = true;
+
+      const submitBtn = e.target.querySelector('button[type="submit"]');
+      const originalText = submitBtn.textContent;
+      submitBtn.textContent = 'Creating...';
+      submitBtn.disabled = true;
+
       const projectPath = document.getElementById('task-project').value;
       const title = document.getElementById('task-title').value;
       const description = document.getElementById('task-description').value;
@@ -626,6 +636,10 @@
       } catch (err) {
         console.error('Create error:', err);
         showToast(err.message || 'Failed to create task', 'error');
+      } finally {
+        isSubmitting = false;
+        submitBtn.textContent = originalText;
+        submitBtn.disabled = false;
       }
     };