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/lib.rs

@@ -173,12 +173,16 @@ fn is_supported(p: &Path) -> bool {
 /// progress. The symptom is a slider that jumps on click but cannot be
 /// dragged, because each move event destroys the thing that would deliver
 /// the next one.
-fn sync_rows(rows: &Rc<slint::VecModel<ParamRow>>, session: &Rc<RefCell<Option<DevelopSession>>>) {
+fn sync_rows(
+    window: &AppWindow,
+    rows: &Rc<slint::VecModel<ParamRow>>,
+    session: &Rc<RefCell<Option<DevelopSession>>>,
+) {
     use slint::Model as _;
 
-    let current = match session.borrow().as_ref() {
-        Some(s) => s.rows(),
-        None => Vec::new(),
+    let (current, samples) = match session.borrow().as_ref() {
+        Some(s) => (s.rows(), s.curve_samples()),
+        None => (Vec::new(), Vec::new()),
     };
 
     if current.len() == rows.row_count() {
@@ -194,6 +198,10 @@ fn sync_rows(rows: &Rc<slint::VecModel<ParamRow>>, session: &Rc<RefCell<Option<D
         // is correct here — there is no drag to preserve.
         rows.set_vec(current);
     }
+
+    // The drawn curve follows the points. Replacing this model wholesale is
+    // safe where replacing `rows` was not: nothing in it is a drag target.
+    window.set_curve_samples(slint::ModelRc::new(slint::VecModel::from(samples)));
 }
 
 /// TRACES: M-13 | M-14
@@ -304,9 +312,13 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
                     // this code names no operation (FR-DEV-3a).
                     match l.session {
                         Some(s) => {
-                            rows.set_vec(s.rows());
                             window.set_adjust_enabled(true);
                             *session.borrow_mut() = Some(s);
+                            // Through `sync_rows` rather than setting rows
+                            // directly, so the curve's drawn shape is
+                            // refreshed by the same path that refreshes the
+                            // controls — one place to keep them in step.
+                            sync_rows(window, &rows, &session);
                             redraw(window);
                         }
                         None => {
@@ -351,7 +363,7 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
             if let Some(s) = session.borrow_mut().as_mut() {
                 s.set_param(op, param, value);
             }
-            sync_rows(&rows, &session);
+            sync_rows(&w, &rows, &session);
             redraw(&w);
         });
     }
@@ -365,7 +377,7 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
             if let Some(s) = session.borrow_mut().as_mut() {
                 s.reset_param(op, param);
             }
-            sync_rows(&rows, &session);
+            sync_rows(&w, &rows, &session);
             redraw(&w);
         });
     }
@@ -379,7 +391,24 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
             if let Some(s) = session.borrow_mut().as_mut() {
                 s.reset_all();
             }
-            sync_rows(&rows, &session);
+            sync_rows(&w, &rows, &session);
+            redraw(&w);
+        });
+    }
+    {
+        // A curve is one control spanning many parameters, so resetting it
+        // clears all of them at once — resetting a single point would leave
+        // a shape the user did not ask for.
+        let weak = window.as_weak();
+        let session = session.clone();
+        let redraw = redraw.clone();
+        let rows = rows.clone();
+        window.on_curve_reset(move |op| {
+            let Some(w) = weak.upgrade() else { return };
+            if let Some(s) = session.borrow_mut().as_mut() {
+                s.reset_curve(op);
+            }
+            sync_rows(&w, &rows, &session);
             redraw(&w);
         });
     }