fix: preserve shared screenshots through sign-in (Closes #741)

src/main.py

@@ -1015,6 +1015,14 @@ def _share_target_login_redirect(request: Request) -> str:
     launch = request.query_params.get("launch", "")
     if launch in {"continue", "new", "agenda"}:
         shared.append(("launch", launch))
+    image_markers = request.query_params.getlist("shared")
+    if image_markers:
+        if (
+            image_markers != ["image"]
+            or request.query_params.getlist("launch") != ["new"]
+        ):
+            return "login"
+        shared.append(("shared", "image"))
     if dashboard_auth.application_path(request) != "/" or not shared:
         return "login"
     continuation = f"./?{urlencode(shared)}"