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>

templates/offline.html

@@ -0,0 +1,19 @@
+{% extends 'base.html' %}
+
+{% block title %}You're Offline - KeepItGoing{% endblock %}
+
+{% block content %}
+<div class="empty-state">
+    <p>You're offline</p>
+    <p class="text-muted">This page isn't available without a connection. Reconnect and try again.</p>
+    <button class="btn btn-primary" onclick="window.location.reload()">Retry</button>
+</div>
+{% endblock %}
+
+{% block auth_content %}
+<div class="empty-state">
+    <p>You're offline</p>
+    <p class="text-muted">This page isn't available without a connection. Reconnect and try again.</p>
+    <button class="btn btn-primary" onclick="window.location.reload()">Retry</button>
+</div>
+{% endblock %}