Fix task edit crashing after due_date/due_time save (regression from reminders)

The web edit/create/quick-add views and the sync endpoint assigned
due_date/due_time/reminder_at straight from request data as raw strings.
Django tolerates this at save() (types get coerced deep in the SQL
layer), but the in-memory instance keeps the raw strings afterward.
Task.reschedule_reminders(), added for per-task reminders and run from
save() on that same instance, was the first code to actually operate on
those fields before a page reload and crashed with a TypeError - which
the service worker's 5xx-treated-as-offline fallback then masked as a
misleading "you're offline" screen instead of a real error.

Fixed by parsing with Django's parse_date/parse_time/parse_datetime at
the point these values are read from the request/payload in
tasks/views.py and sync/views.py, rather than trusting raw strings.

5 new regression tests exercise the actual views/endpoint with raw
date/time strings and assert proper types land on the model.

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

tasks/tests.py

@@ -133,3 +133,47 @@ class IsOverdueTests(TestCase):
         mock_now.return_value = django_timezone.make_aware(datetime(2026, 1, 15, 23, 0, 0))
         task = self.make_task(due_date=date(2026, 1, 1), due_time=dt_time(9, 0, 0), status='completed')
         self.assertFalse(task.is_overdue)
+
+
+class WebFormDueDateTypeTests(TestCase):
+    """
+    The web views assign due_date/due_time straight from request.POST (raw
+    strings), relying on Django to coerce them at the DB layer - which
+    leaves the in-memory instance holding strings after save(). Anything
+    that touches those fields on that same instance afterward (e.g.
+    Task.reschedule_reminders(), called from save() itself) must not choke
+    on that. Regression test for a real production crash on task edit.
+    """
+
+    def setUp(self):
+        self.user = User.objects.create_user(
+            username='webformuser', email='webformuser@example.com', password='testpass123',
+        )
+        self.client.force_login(self.user)
+
+    def test_editing_due_date_and_time_does_not_crash(self):
+        task = Task.objects.create(user=self.user, title='Task')
+        response = self.client.post(f'/tasks/{task.id}/', {
+            'title': 'Task', 'status': 'pending', 'priority': 'medium',
+            'due_date': '2026-09-10', 'due_time': '17:00', 'recurrence': 'none',
+        })
+        self.assertEqual(response.status_code, 302)
+        task.refresh_from_db()
+        self.assertEqual(task.due_date, date(2026, 9, 10))
+        self.assertEqual(task.due_time, dt_time(17, 0))
+
+    def test_creating_task_with_due_date_does_not_crash(self):
+        response = self.client.post('/tasks/new/', {
+            'title': 'New task', 'status': 'pending', 'priority': 'medium',
+            'due_date': '2026-09-10', 'due_time': '17:00', 'recurrence': 'none',
+        })
+        self.assertEqual(response.status_code, 302)
+        task = Task.objects.get(title='New task')
+        self.assertEqual(task.due_date, date(2026, 9, 10))
+        self.assertEqual(task.due_time, dt_time(17, 0))
+
+    def test_quick_add_task_with_due_date_does_not_crash(self):
+        response = self.client.post('/tasks/quick-add/', {'title': 'Quick task', 'due_date': '2026-09-10'})
+        self.assertEqual(response.status_code, 302)
+        task = Task.objects.get(title='Quick task')
+        self.assertEqual(task.due_date, date(2026, 9, 10))