fix: remove duplicate submit handler that caused double task creation

The Save button already has type=submit which naturally triggers
form submission. The additional click handler dispatching submit
event caused handleTaskSubmit to run twice.

templates/tasks.html

@@ -75,7 +75,7 @@
             </div>
             <div id="task-error" class="error-message"></div>
             <div class="modal-actions">
-                <button type="submit" class="btn-primary" id="task-save-btn">Save</button>
+                <button type="submit" class="btn-primary">Save</button>
                 <button type="button" class="btn-secondary" id="task-modal-cancel">Cancel</button>
             </div>
         </form>