Add Web Push notifications (PWA)

Wires up the previously-scaffolded VAPID/DeviceToken infrastructure end
to end: browser subscription flow on the Profile page's existing "Push
Notifications" toggle, a service worker push/notificationclick handler,
and server-side sending from the daily task digest. Also broadens that
digest's eligibility query so push-only users (email notifications off)
aren't silently skipped, and adds `generate_vapid_keys` since the pinned
py-vapid's own key generator is broken against current cryptography.

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

templates/base.html

@@ -147,7 +147,7 @@
     {% endif %}
     {% endblock %}
 
-    <script src="{% static 'js/app.js' %}?v=4"></script>
+    <script src="{% static 'js/app.js' %}?v=6"></script>
     {% block extra_js %}{% endblock %}
 </body>
 </html>

templates/sw.js

@@ -47,3 +47,20 @@ self.addEventListener('fetch', (event) => {
         );
     }
 });
+
+self.addEventListener('push', (event) => {
+    const data = event.data ? event.data.json() : {};
+    event.waitUntil(
+        self.registration.showNotification(data.title || 'KeepItGoing', {
+            body: data.body || '',
+            icon: '{% static "favicons/icon-192.png" %}',
+            badge: '{% static "favicons/icon-192.png" %}',
+            data: { url: data.url || '/' },
+        })
+    );
+});
+
+self.addEventListener('notificationclick', (event) => {
+    event.notification.close();
+    event.waitUntil(clients.openWindow(event.notification.data.url || '/'));
+});

templates/users/profile.html

@@ -18,7 +18,7 @@
 <div class="card" style="background: var(--surface); padding: 1.5rem; border-radius: 0.75rem; margin-bottom: 1.5rem; border: 1px solid var(--border);">
     <h2 style="font-size: 1.25rem; font-weight: 600; margin-bottom: 1.5rem;">Personal Information</h2>
 
-    <form method="post">
+    <form method="post" onsubmit="handleProfileFormSubmit(event, this)">
         {% csrf_token %}
 
         <div style="display: grid; grid-template-columns: 1fr 1fr; gap: 1.5rem; margin-bottom: 1.5rem;">
@@ -96,6 +96,7 @@
             <label style="display: flex; align-items: center; cursor: pointer; padding: 0.75rem; background: var(--surface); border: 1px solid var(--border); border-radius: 0.375rem;">
                 <input type="checkbox"
                        name="push_notifications"
+                       data-vapid-public-key="{{ vapid_public_key }}"
                        {% if user.push_notifications %}checked{% endif %}
                        style="margin-right: 0.75rem; width: 1rem; height: 1rem;">
                 <div>