Fix layout: move create button to header, increase description area height

- Move '+ Create Task' button into tasks-header to avoid breaking scroll/edit height alignment
- Increase edit-task description textarea from rows=3 to rows=6
- Remove unused .create-task-container and #create-task-btn CSS rules

static/css/tasks.css

@@ -198,10 +198,6 @@
     margin-top: 1rem;
 }
 
-#create-task-btn {
-    margin-top: 1rem;
-}
-
 @media (orientation: portrait), (max-width: 1023px) {
     .side-panel {
         position: fixed;
@@ -272,10 +268,6 @@
     .side-panel-close {
         display: none;
     }
-
-    .create-task-container {
-        flex-shrink: 0;
-    }
 }
 
 .empty-state {