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>

static/js/app.js

@@ -8,6 +8,11 @@ document.addEventListener('DOMContentLoaded', function() {
     initTaskSelection();
     initTimerDisplays();
     registerServiceWorker();
+    maybeBackgroundSync();
+});
+
+window.addEventListener('online', function() {
+    runBackgroundSync();
 });
 
 /* ============================================