Let the top level be chosen as the library root

Confirming "/" in the folder picker set an empty root, which the launch
model read as no root at all: "Open library" stayed disabled after the
question had plainly been answered, and a folder library — whose folder
is the whole library — could never be opened without first descending
into a subfolder of it. The empty string was carrying two meanings.

Record the choice as its own fact on the account (`root_chosen`, defaulted
so existing configuration loads unchanged), treat a folder endpoint as
chosen by definition, and let the launch screen say so: a folder is shown
as a LIBRARY rather than an ACCOUNT, the second question becomes an
optional "scan only a subfolder", and the library header names the folder
instead of calling it "· whole account".

ui/dr-ui/src/launch_ui.rs

@@ -44,7 +44,8 @@ pub fn render(window: &AppWindow, controller: &LaunchController) {
 
     window.set_launch_signed_in(m.is_signed_in());
     window.set_launch_account(m.account_label().into());
-    window.set_launch_root(m.library_root().into());
+    window.set_launch_root(m.library_root_label().into());
+    window.set_launch_endpoint_is_library(m.endpoint_is_library());
     window.set_launch_server(m.server_url.clone().into());
     window.set_launch_folder(m.folder_path.clone().into());
     window.set_launch_busy(m.is_busy());