feat: v4.0.2 — qualité & robustesse (session expiry UX, validation, mobile, tests)

- Session expiry: redirects ajoutent ?expired=1 → bannière "Session expired" sur login
- Page titles: titre vide → 'Untitled' par défaut (au lieu de chaîne vide)
- Error handling: try/catch dans create_page avec message user-friendly (500)
- Mobile responsive: sidebar slide-in, modales centrées, landing page adaptative
- 10 nouveaux tests de non-régression: landing, register, 404, validation, duplicate
- 143 tests passent (133 + 10)

app/routers/dashboard.py

@@ -27,7 +27,7 @@ def _get_user_or_redirect(request: Request):
                     return {"id": 1, "login": "admin", "full_name": "Admin", "is_admin": True}
         except Exception:
             pass
-        return RedirectResponse("/auth/login?provider=local", status_code=302)
+        return RedirectResponse("/auth/login?provider=local&expired=1", status_code=302)
     return user
 
 
@@ -1285,7 +1285,7 @@ async def workspaces_page(request: Request):
     sidebar = _sidebar_data(request, [], include_workspace=False)
     user = SessionManager.decode_session(request.cookies.get("flowdeck_session", ""))
     if not user:
-        return RedirectResponse("/auth/login?provider=local", status_code=302)
+        return RedirectResponse("/auth/login?provider=local&expired=1", status_code=302)
     ctx = {**sidebar, "user": user}
     # Pass active workspace for breadcrumb nav menu (null on workspaces home)
     ws = _get_active_workspace(request, user_id=_get_user_id(request))