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/library_ui.rs

@@ -984,14 +984,7 @@ pub fn open(
     window.set_library_status("Starting…".into());
     // Always visible: two folders one letter apart are easy to confuse, and a
     // scan of the wrong one is indistinguishable from a broken scan.
-    window.set_library_root_label(
-        if conn.account.root.is_empty() {
-            format!("{} · whole account", conn.account.user_id)
-        } else {
-            format!("{}/{}", conn.account.user_id, conn.account.root)
-        }
-        .into(),
-    );
+    window.set_library_root_label(library_root_label(&conn.account).into());
 
     // An empty filter would walk the whole tree and match nothing, which looks
     // exactly like a broken scan. Say so instead.
@@ -7158,10 +7151,49 @@ fn stop(slot: &RefCell<Option<slint::Timer>>) {
     }
 }
 
+/// The line in the library header that says what is being catalogued.
+///
+/// For an account with a user, the user and the chosen subtree, or the
+/// whole account when none was chosen. For a folder there is no user and
+/// the endpoint is the library, so the folder's own name: `library`, or
+/// `library/2026` when narrowed — never " · whole account", which is a
+/// sentence about a server said of a directory.
+fn library_root_label(account: &Account) -> String {
+    if account.login.is_empty() {
+        let folder = std::path::Path::new(&account.endpoint)
+            .file_name()
+            .map(|n| n.to_string_lossy().into_owned())
+            .unwrap_or_else(|| account.endpoint.clone());
+        if account.root.is_empty() {
+            folder
+        } else {
+            format!("{folder}/{}", account.root)
+        }
+    } else if account.root.is_empty() {
+        format!("{} · whole account", account.user_id)
+    } else {
+        format!("{}/{}", account.user_id, account.root)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn the_header_names_a_folder_library_by_its_folder() {
+        let mut folder = Account::new("folder", "/var/tmp/dr-demo/library");
+        assert_eq!(library_root_label(&folder), "library");
+        folder.root = "2026".into();
+        assert_eq!(library_root_label(&folder), "library/2026");
+
+        let mut cloud =
+            Account::new("nextcloud", "https://cloud.example").with_login("duncan", "duncan");
+        assert_eq!(library_root_label(&cloud), "duncan · whole account");
+        cloud.root = "PhotosRaw".into();
+        assert_eq!(library_root_label(&cloud), "duncan/PhotosRaw");
+    }
+
     #[test]
     fn a_scrub_fraction_interpolates_within_the_span() {
         // The point of going fractional: a slow drag must advance