Make "Choose folder" an actual folder picker

It previously fetched the folder list and threw it away into a status
line — a button that looked like it worked and did not. Now it opens a
browsable picker: click a folder to descend, ".." to go back, "Use this
folder" to select, "Cancel" to leave the root unchanged.

Descends one level per click because that is what the backend supports:
Depth: infinity is frequently disabled server-side and prohibitively
expensive where it is not (ARCH §8.4).

The chosen root persists immediately on confirm, so it survives a crash
before the library is opened. Confirming at the account root is allowed —
a user may legitimately keep everything at the top level — and cancelling
leaves any previous selection untouched, which a test asserts.

Verified against nextcloud.tourolle.paris at both depths: 30 folders at
the root, 21 year-folders inside PhotosRaw.

19 launch tests, 38 in dr-ui.

ui/dr-ui/src/launch_ui.rs

@@ -66,6 +66,20 @@ pub fn render(window: &AppWindow, controller: &LaunchController) {
     window.set_launch_status(status.unwrap_or_default().into());
     window.set_launch_error(m.error.clone().unwrap_or_default().into());
 
+    // Folder picker.
+    let browsing = m.browser.is_some();
+    window.set_launch_browsing(browsing);
+    if let Some(b) = &m.browser {
+        window.set_launch_browse_path(b.path.clone().into());
+        window.set_launch_browse_loading(b.loading);
+        let entries: Vec<slint::SharedString> = b
+            .entries
+            .iter()
+            .map(|e| slint::SharedString::from(e.as_str()))
+            .collect();
+        window.set_launch_browse_entries(slint::ModelRc::new(slint::VecModel::from(entries)));
+    }
+
     let labels: Vec<slint::SharedString> = m
         .formats
         .iter()
@@ -148,9 +162,9 @@ where
         let ctl = controller.clone();
         window.on_launch_choose_folder(move || {
             let Some(w) = weak.upgrade() else { return };
-            ctl.model.borrow_mut().busy("Listing folders…");
+            ctl.model.borrow_mut().open_browser();
             render(&w, &ctl);
-            spawn_folder_list(w.as_weak(), ctl.clone());
+            spawn_folder_list(w.as_weak(), ctl.clone(), String::new());
         });
     }
 
@@ -168,6 +182,66 @@ where
         });
     }
 
+    // --- folder picker navigation ---------------------------------------
+    {
+        let weak = window.as_weak();
+        let ctl = controller.clone();
+        window.on_launch_browse_into(move |name| {
+            let Some(w) = weak.upgrade() else { return };
+            let target = {
+                let m = ctl.model.borrow();
+                m.browser.as_ref().map(|b| b.child_path(&name))
+            };
+            if let Some(path) = target {
+                ctl.model.borrow_mut().browse_to(path.clone());
+                render(&w, &ctl);
+                spawn_folder_list(w.as_weak(), ctl.clone(), path);
+            }
+        });
+    }
+    {
+        let weak = window.as_weak();
+        let ctl = controller.clone();
+        window.on_launch_browse_up(move || {
+            let Some(w) = weak.upgrade() else { return };
+            let parent = {
+                let m = ctl.model.borrow();
+                m.browser.as_ref().and_then(|b| b.parent_path())
+            };
+            if let Some(path) = parent {
+                ctl.model.borrow_mut().browse_to(path.clone());
+                render(&w, &ctl);
+                spawn_folder_list(w.as_weak(), ctl.clone(), path);
+            }
+        });
+    }
+    {
+        let weak = window.as_weak();
+        let ctl = controller.clone();
+        window.on_launch_browse_confirm(move || {
+            let Some(w) = weak.upgrade() else { return };
+            let chosen = ctl.model.borrow_mut().choose_current_folder();
+            if let Some(session) = chosen {
+                // Persist immediately: a chosen root must survive a crash
+                // before the library is opened.
+                if let Err(e) = ctl.store.update(&session) {
+                    log::warn!("saving library root: {e}");
+                }
+                log::info!("library root set to /{}", session.root);
+            }
+            render(&w, &ctl);
+        });
+    }
+    {
+        let weak = window.as_weak();
+        let ctl = controller.clone();
+        window.on_launch_browse_cancel(move || {
+            let Some(w) = weak.upgrade() else { return };
+            ctl.model.borrow_mut().close_browser();
+            render(&w, &ctl);
+        });
+    }
+
     // --- copy the login URL ---------------------------------------------
     {
         let ctl = controller.clone();
@@ -301,7 +375,7 @@ fn poll_channel(
 }
 
 /// List top-level folders so one can be chosen as the library root.
-fn spawn_folder_list(weak: slint::Weak<AppWindow>, ctl: Rc<LaunchController>) {
+fn spawn_folder_list(weak: slint::Weak<AppWindow>, ctl: Rc<LaunchController>, path: String) {
     let Some(session) = ctl.model.borrow().session().cloned() else {
         return;
     };
@@ -329,7 +403,7 @@ fn spawn_folder_list(weak: slint::Weak<AppWindow>, ctl: Rc<LaunchController>) {
         };
         rt.block_on(async {
             match NextcloudBackend::new(&creds, &user_id) {
-                Ok(b) => match b.list(&RemotePath::root(), None).await {
+                Ok(b) => match b.list(&RemotePath::new(&path), None).await {
                     Ok(entries) => {
                         let mut dirs: Vec<String> = entries
                             .iter()