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/views.py

@@ -7,6 +7,7 @@ from django.contrib.auth.decorators import login_required
 from django.db.models import Q
 from django.shortcuts import render, redirect, get_object_or_404
 from django.utils import timezone
+from django.utils.dateparse import parse_date, parse_time
 from django.utils.http import url_has_allowed_host_and_scheme
 from django.views import View
 from django.views.decorators.http import require_POST
@@ -466,10 +467,10 @@ class TaskDetailView(View):
             task.priority = priority
 
         due_date = request.POST.get('due_date')
-        task.due_date = due_date if due_date else None
+        task.due_date = parse_date(due_date) if due_date else None
 
         due_time = request.POST.get('due_time')
-        task.due_time = due_time if due_time else None
+        task.due_time = parse_time(due_time) if due_time else None
 
         # Validate recurrence against allowed choices
         recurrence = request.POST.get('recurrence', 'none')
@@ -540,8 +541,8 @@ class TaskCreateView(View):
             description=request.POST.get('description', ''),
             status=status,
             priority=priority,
-            due_date=request.POST.get('due_date') or None,
-            due_time=request.POST.get('due_time') or None,
+            due_date=parse_date(request.POST.get('due_date')) if request.POST.get('due_date') else None,
+            due_time=parse_time(request.POST.get('due_time')) if request.POST.get('due_time') else None,
             recurrence=recurrence,
             recurrence_rule=request.POST.get('recurrence_rule', '') if recurrence == 'custom' else '',
         )
@@ -559,7 +560,7 @@ def task_quick_add(request):
     task = Task.objects.create(
         user=request.user,
         title=request.POST.get('title'),
-        due_date=request.POST.get('due_date') or None,
+        due_date=parse_date(request.POST.get('due_date')) if request.POST.get('due_date') else None,
     )
     # Handle optional tag
     tag_id = request.POST.get('tag')