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/views.py

@@ -8,7 +8,7 @@ import logging
 import uuid
 from datetime import datetime
 from django.utils import timezone
-from django.utils.dateparse import parse_datetime
+from django.utils.dateparse import parse_date, parse_datetime, parse_time
 from rest_framework import status
 from rest_framework.decorators import api_view, permission_classes, throttle_classes
 from rest_framework.permissions import IsAuthenticated
@@ -289,6 +289,14 @@ def process_time_entry_changes(user, entry_changes, last_sync_at):
     return conflicts
 
 
+def _parsed(data, key, default, parser):
+    """Get data[key], parsing it if it's still a raw string (e.g. from JSON), else default if absent."""
+    if key not in data:
+        return default
+    value = data[key]
+    return parser(value) if isinstance(value, str) else value
+
+
 def update_task_from_data(task, data):
     """Update a task from sync data."""
     # Track old status to detect completion
@@ -298,9 +306,9 @@ def update_task_from_data(task, data):
     task.description = data.get('description', task.description)
     task.status = data.get('status', task.status)
     task.priority = data.get('priority', task.priority)
-    task.due_date = data.get('due_date', task.due_date)
-    task.due_time = data.get('due_time', task.due_time)
-    task.reminder_at = data.get('reminder_at', task.reminder_at)
+    task.due_date = _parsed(data, 'due_date', task.due_date, parse_date)
+    task.due_time = _parsed(data, 'due_time', task.due_time, parse_time)
+    task.reminder_at = _parsed(data, 'reminder_at', task.reminder_at, parse_datetime)
     task.recurrence = data.get('recurrence', task.recurrence)
     task.recurrence_rule = data.get('recurrence_rule', task.recurrence_rule)
     task.sort_order = data.get('sort_order', task.sort_order)
@@ -341,9 +349,9 @@ def create_task_from_data(user, data):
         description=data.get('description', ''),
         status=data.get('status', 'pending'),
         priority=data.get('priority', 'medium'),
-        due_date=data.get('due_date'),
-        due_time=data.get('due_time'),
-        reminder_at=data.get('reminder_at'),
+        due_date=_parsed(data, 'due_date', None, parse_date),
+        due_time=_parsed(data, 'due_time', None, parse_time),
+        reminder_at=_parsed(data, 'reminder_at', None, parse_datetime),
         recurrence=data.get('recurrence', 'none'),
         recurrence_rule=data.get('recurrence_rule', ''),
         sort_order=data.get('sort_order', 0),