Pure useShape API for mutations, event lifecycle e2e tests

Refactor FestipodDataContext to use only the useShape ORM API:
- Remove ng.sparql_update, sessionPromise, ensureGraphNuri imports
- Use privateNuri (useShape scope) directly as @graph for adds
- Create/join are now synchronous (no async wrapper needed)
- Leave uses ngSet.delete() — known limitation: doesn't persist (@wip)

Add event lifecycle e2e scenarios (cycle-de-vie-evenement.feature):
- Create event via form and verify on home screen
- Created event persists after reconnexion
- Consult event detail from home
- Join an event
- Modify event location and verify
- Leave + persistence tagged @wip (ngSet.delete doesn't persist)

Fix DemoMode external navigation: sync initialScreenId prop changes
to internal state via useEffect (was ignored after first mount).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/components/DemoMode.tsx

@@ -28,6 +28,15 @@ export function DemoMode({ initialScreenId, onBack, onNavigateToStory }: DemoMod
   const [sidebarOpen, setSidebarOpen] = useState(false);
   const isMobile = useIsMobile();
 
+  // Sync with external hash navigation (e.g. e2e tests changing window.location.hash)
+  useEffect(() => {
+    if (initialScreenId !== currentScreenId) {
+      setCurrentScreenId(initialScreenId);
+      setHistory(prev => [...prev, initialScreenId]);
+      setHistoryIndex(prev => prev + 1);
+    }
+  }, [initialScreenId]);
+
   const currentScreen = getScreen(currentScreenId);
   const ScreenComponent = currentScreen?.component;
   const linkedStories = getStoriesForScreen(currentScreenId);