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>

static/js/app.js

@@ -7,8 +7,19 @@ document.addEventListener('DOMContentLoaded', function() {
     initTheme();
     initTaskSelection();
     initTimerDisplays();
+    registerServiceWorker();
 });
 
+/* ============================================
+   Service Worker
+   ============================================ */
+
+function registerServiceWorker() {
+    if ('serviceWorker' in navigator) {
+        navigator.serviceWorker.register('/sw.js');
+    }
+}
+
 /* ============================================
    Theme Toggle
    ============================================ */