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.

core/dr-gpu/examples/develop.rs

@@ -12,7 +12,7 @@
 //! it. This is a diagnostic, not the export path (FR-EXP-*).
 
 use dr_gpu::{AdjustPass, Demosaicer, GpuContext};
-use dr_pipeline::ops::{colour, colour_mixer, contrast, exposure, tone, white_balance};
+use dr_pipeline::ops::{colour, colour_mixer, contrast, curve, exposure, tone, white_balance};
 use dr_pipeline::{EditGraph, ParamId};
 
 fn main() {
@@ -86,6 +86,16 @@ fn main() {
         "mixer_one" => {
             graph.set_param(colour_mixer::ID, ParamId("green_sat"), 100.0);
         }
+        // A classic S-curve: shadows down, highlights up, mid held.
+        "curve_s" => {
+            graph.set_param(curve::ID, curve::P1_Y, 0.15);
+            graph.set_param(curve::ID, curve::P3_Y, 0.85);
+        }
+        // The inverse, a film-like lifted-shadow look.
+        "curve_lift" => {
+            graph.set_param(curve::ID, curve::P0_Y, 0.12);
+            graph.set_param(curve::ID, curve::P1_Y, 0.32);
+        }
         _ => {}
     }