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/routers/dashboard.py

@@ -554,6 +554,16 @@ async def help_page(request: Request):
 <li><b>My Tasks</b> — Aggregated tasks from all collections</li>
 </ul>
 </div>
+
+<div class="help-card">
+<h3><span class="icon">📶</span>Offline & PWA</h3>
+<p>Install FlowDeck as an app and keep working without a connection.</p>
+<ul>
+<li><b>Install</b> — browser menu → <i>Install app</i> / <i>Add to Home Screen</i></li>
+<li>Edits made offline are queued locally and synced automatically</li>
+<li>A <b>⟳</b> marker shows pages with pending changes</li>
+</ul>
+</div>
 </div>
 
 <div class="help-section">
@@ -579,6 +589,25 @@ FlowDeck supports three authentication methods:<br>
 </p>
 </div>
 
+<div class="help-section">
+<h2>📶 Offline mode (PWA)</h2>
+<p style="color:var(--text-dim);font-size:14px;line-height:1.6;">
+FlowDeck is a Progressive Web App: pages you visited stay available offline and your
+edits are saved locally, then synchronised when the connection returns.<br><br>
+<b>Install:</b> open your browser menu and choose <i>Install app</i> (Chrome/Edge) or
+<i>Add to Home Screen</i> (Safari/iOS). FlowDeck then opens in its own window.<br>
+<b>Offline editing:</b> while offline, the editor stores changes in the browser
+(IndexedDB) and shows an offline banner with the number of pending changes. A
+<b>⟳</b> icon appears next to pages that have unsynced edits.<br>
+<b>Reconnection:</b> the queue is replayed automatically (and via Background Sync).
+A spinner badge appears while syncing, followed by a confirmation toast.<br>
+<b>Conflicts:</b> if a page changed on the server, the latest edit wins and a notice is
+shown. If a page was deleted server-side, your offline copy is recreated as an orphan
+page. If a page with the same title already exists, the offline copy is renamed
+<i>“Title (copie offline)”</i>.
+</p>
+</div>
+
 <div class="help-section">
 <h2>💡 Tips</h2>
 <p style="color:var(--text-dim);font-size:14px;line-height:1.6;">