Make the web app installable as a PWA (Tier 1: app shell + offline fallback)

Adds a manifest, service worker, and branded icons so the site can be
installed to a home screen/desktop, plus an offline fallback page so a
dropped connection shows something friendlier than the browser's default
error. Icons are rasterized from the existing favicon.svg mark via
rsvg-convert. The manifest and service worker are served through small
Django views (not raw static files) so their asset URLs pick up
WhiteNoise's content-hashed filenames in prod/selfhosted, and the service
worker is served from the site root so its scope covers the whole app.

Does not include true offline task data or Web Push notifications --
those are tracked separately as larger follow-up projects.

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

config/urls.py

@@ -6,18 +6,24 @@ from django.contrib import admin
 from django.urls import path, include
 from django.conf import settings
 from django.conf.urls.static import static
+from django.views.generic import TemplateView
 
 from users.urls import api_urlpatterns as users_api_urls, web_urlpatterns as users_web_urls
 from tasks.urls import api_urlpatterns as tasks_api_urls, web_urlpatterns as tasks_web_urls
 from sync.urls import api_urlpatterns as sync_api_urls
 from notifications.urls import api_urlpatterns as notifications_api_urls
 from tasks.views_debug import debug_user_agent
-from .views import api_root
+from .views import api_root, manifest_webmanifest, service_worker
 
 urlpatterns = [
     # Admin
     path('admin/', admin.site.urls),
 
+    # PWA
+    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'),
+
     # Debug endpoint (remove in production)
     path('debug/user-agent/', debug_user_agent, name='debug-user-agent'),