Add offline task CRUD, phase 1 (PWA)

Lets users create, edit, and complete tasks with no network connection,
queued locally in IndexedDB and replayed via the existing /api/sync/
protocol once back online -- the same protocol the native Android app
already uses, with zero backend changes. The service worker now routes
the dashboard's offline fallback to a small client-rendered task app
instead of the generic "you're offline" page; every other route keeps
the generic fallback.

Conflicts (server row touched elsewhere since last sync) auto-resolve
as "local wins" rather than surfacing a resolution UI. Scope is title/
status/priority/due-date only -- tags, subtasks, time tracking, and
recurrence editing offline are out of scope for this phase.

Also adds the first real test coverage for sync/views.py (previously
untested): new-item creation, soft-delete-bypasses-conflict-check,
conflict detection/discarding, and both resolve_conflict paths.

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

config/urls.py

@@ -23,6 +23,7 @@ urlpatterns = [
     path('manifest.webmanifest', manifest_webmanifest, name='manifest'),
     path('sw.js', service_worker, name='service-worker'),
     path('offline/', TemplateView.as_view(template_name='offline.html'), name='offline'),
+    path('offline/tasks/', TemplateView.as_view(template_name='offline_tasks.html'), name='offline-tasks'),
 
     # Debug endpoint (remove in production)
     path('debug/user-agent/', debug_user_agent, name='debug-user-agent'),