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/views.py

@@ -3,6 +3,23 @@ API root views for KeepItGoing.
 """
 
 from django.http import JsonResponse
+from django.shortcuts import render
+
+
+def manifest_webmanifest(request):
+    """Serve the PWA web app manifest, rendered so icon URLs pick up static hashing."""
+    return render(request, 'manifest.webmanifest', content_type='application/manifest+json')
+
+
+def service_worker(request):
+    """
+    Serve the service worker script from the site root so its default scope
+    covers the whole app (a service worker can only control paths at or
+    below where it's served from).
+    """
+    response = render(request, 'sw.js', content_type='application/javascript')
+    response['Cache-Control'] = 'no-cache'
+    return response
 
 
 def api_root(request):