Add offline task CRUD, phase 1 (PWA)

Lets users create, edit, and complete tasks with no network connection,
queued locally in IndexedDB and replayed via the existing /api/sync/
protocol once back online -- the same protocol the native Android app
already uses, with zero backend changes. The service worker now routes
the dashboard's offline fallback to a small client-rendered task app
instead of the generic "you're offline" page; every other route keeps
the generic fallback.

Conflicts (server row touched elsewhere since last sync) auto-resolve
as "local wins" rather than surfacing a resolution UI. Scope is title/
status/priority/due-date only -- tags, subtasks, time tracking, and
recurrence editing offline are out of scope for this phase.

Also adds the first real test coverage for sync/views.py (previously
untested): new-item creation, soft-delete-bypasses-conflict-check,
conflict detection/discarding, and both resolve_conflict paths.

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

templates/sw.js

@@ -1,15 +1,19 @@
-{% load static %}const CACHE_NAME = 'keepitgoing-shell-v1';
+{% load static %}const CACHE_NAME = 'keepitgoing-shell-v2';
 const OFFLINE_URL = '{% url "offline" %}';
+const OFFLINE_TASKS_URL = '{% url "offline-tasks" %}';
 
 const PRECACHE_URLS = [
     "{% static 'css/app.css' %}",
     "{% static 'js/app.js' %}",
+    "{% static 'js/offline-db.js' %}",
+    "{% static 'js/offline-tasks.js' %}",
     "{% static 'favicons/favicon.svg' %}",
     "{% static 'favicons/icon-192.png' %}",
     "{% static 'favicons/icon-512.png' %}",
     "{% static 'favicons/icon-512-maskable.png' %}",
     "{% static 'favicons/apple-touch-icon-180.png' %}",
     OFFLINE_URL,
+    OFFLINE_TASKS_URL,
 ];
 
 self.addEventListener('install', (event) => {
@@ -32,9 +36,14 @@ self.addEventListener('fetch', (event) => {
     const request = event.request;
 
     // Navigations: try the network first, fall back to the offline page.
+    // The dashboard route falls back to the offline-capable task app instead
+    // of the generic offline page, since it can still show/edit cached tasks.
     if (request.mode === 'navigate') {
         event.respondWith(
-            fetch(request).catch(() => caches.match(OFFLINE_URL))
+            fetch(request).catch(() => {
+                const path = new URL(request.url).pathname;
+                return caches.match(path === '/' ? OFFLINE_TASKS_URL : OFFLINE_URL);
+            })
         );
         return;
     }