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>

sync/tests.py

@@ -171,3 +171,55 @@ class SyncEndpointTests(TestCase):
         self.assertEqual(entry.notes, 'local notes')
         conflict.refresh_from_db()
         self.assertEqual(conflict.status, 'resolved_local')
+
+    def test_creating_task_with_due_date_and_time_does_not_crash(self):
+        """
+        Sync payloads carry due_date/due_time as JSON strings. Regression
+        test: these must end up as real date/time objects on the created
+        Task, not raw strings left for Task.reschedule_reminders() (called
+        from save()) to choke on.
+        """
+        new_sync_id = str(uuid.uuid4())
+        response = self.client.post('/api/sync/', {
+            'device_id': 'web-test-device',
+            'last_sync_token': None,
+            'changes': {
+                'tasks': [{
+                    'sync_id': new_sync_id,
+                    'title': 'Due-dated task',
+                    'status': 'pending',
+                    'priority': 'medium',
+                    'due_date': '2026-09-10',
+                    'due_time': '17:00:00',
+                }],
+                'tags': [],
+                'time_entries': [],
+            },
+        }, format='json')
+
+        self.assertEqual(response.status_code, 200)
+        task = Task.objects.get(sync_id=new_sync_id)
+        self.assertEqual(task.due_date.isoformat(), '2026-09-10')
+        self.assertEqual(task.due_time.isoformat(), '17:00:00')
+
+    def test_updating_task_due_date_and_time_does_not_crash(self):
+        task = Task.objects.create(user=self.user, title='Task to update')
+
+        response = self.client.post('/api/sync/', {
+            'device_id': 'web-test-device',
+            'last_sync_token': None,
+            'changes': {
+                'tasks': [{
+                    'sync_id': str(task.sync_id),
+                    'due_date': '2026-09-10',
+                    'due_time': '17:00:00',
+                }],
+                'tags': [],
+                'time_entries': [],
+            },
+        }, format='json')
+
+        self.assertEqual(response.status_code, 200)
+        task.refresh_from_db()
+        self.assertEqual(task.due_date.isoformat(), '2026-09-10')
+        self.assertEqual(task.due_time.isoformat(), '17:00:00')