Default task sort to due date instead of raw model order

The "Default" option cleared the sort param and fell back to the
model's Meta.ordering (sort_order, -priority, due_date, created_at),
which read as broken/unsorted to users. Default is now due date
(earliest first), matching the existing due_date sort option.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/tasks/dashboard.html

@@ -9,7 +9,6 @@
     <div style="display: flex; align-items: center; gap: var(--space-md);">
         <span class="task-count">{{ tasks|length }} task{{ tasks|length|pluralize }}</span>
         <select id="sort-select" onchange="updateSort()" style="padding: 4px 8px; border: 1px solid var(--border); border-radius: var(--radius-sm); background: var(--surface); color: var(--text);">
-            <option value="default" {% if current_sort == 'default' %}selected{% endif %}>Default</option>
             <option value="due_date" {% if current_sort == 'due_date' %}selected{% endif %}>Due Date (Earliest)</option>
             <option value="due_date_desc" {% if current_sort == 'due_date_desc' %}selected{% endif %}>Due Date (Latest)</option>
             <option value="priority" {% if current_sort == 'priority' %}selected{% endif %}>Priority (High to Low)</option>
@@ -76,7 +75,7 @@ if (timerDisplay) {
 function updateSort() {
     const sortValue = document.getElementById('sort-select').value;
     const url = new URL(window.location.href);
-    if (sortValue === 'default') {
+    if (sortValue === 'due_date') {
         url.searchParams.delete('sort');
     } else {
         url.searchParams.set('sort', sortValue);