feat: v6.0.0 PWA offline support

- manifest + icones, service worker (precache, network-first, Background Sync)

- module client FlowOffline (IndexedDB, queue, delta, flush) + hook editeur

- endpoints /api/v2/sync/{delta,batch,status} + moteur de sync (conflits LWW/orpheline/copie offline)

- migrations offline_sync_queue + sync_version (triggers)

- UI offline (banner, badge sync, toasts, icone dirty) + doc /help

- tests pytest (sync, migrations, SW, offline) + E2E Playwright; bump 6.0.0

app/templates/landing.html

@@ -6,6 +6,11 @@
   {% from '_icons.html' import fd_icon %}
   <title>FlowDeck — All-in-one workspace</title>
   <link rel="icon" type="image/svg+xml" href="/static/favicon.svg">
+  <link rel="manifest" href="/static/manifest.json">
+  <meta name="theme-color" content="#191919">
+  <link rel="apple-touch-icon" href="/static/icons/apple-touch-icon.png">
+  <meta name="apple-mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
   <style>
     :root {
       --bg: #191919;
@@ -201,5 +206,14 @@
   FlowDeck v4.0.1 — Open source · Self-hosted · Notion-compatible
 </footer>
 
+<script data-cfasync="false">
+(function() {
+  if (!('serviceWorker' in navigator)) return;
+  window.addEventListener('load', function() {
+    navigator.serviceWorker.register('/sw.js').catch(function() {});
+  });
+})();
+</script>
+
 </body>
 </html>