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/ui/app.slint

@@ -192,6 +192,7 @@ export component AppWindow inherits Window {
     in property <bool> launch-signed-in: false;
     in property <string> launch-account: "";
     in property <string> launch-root: "";
+    in property <bool> launch-endpoint-is-library: false;
     in property <string> launch-server: "";
     in property <string> launch-folder: "";
     in property <bool> launch-busy: false;
@@ -1546,6 +1547,7 @@ in property <bool> panel-visible: true;
             signed-in: root.launch-signed-in;
             account: root.launch-account;
             library-root: root.launch-root;
+            endpoint-is-library: root.launch-endpoint-is-library;
             server-url: root.launch-server;
             folder-path: root.launch-folder;
             busy: root.launch-busy;