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);
+        }
         _ => {}
     }
 

core/dr-gpu/src/adjust.rs

@@ -736,9 +736,20 @@ mod tests {
 
         let mut g = EditGraph::default_chain();
         for cap in EditGraph::default_chain().capabilities() {
-            for p in &cap.params {
-                if let dr_pipeline::ParamKind::Scalar { max, .. } = p.kind {
-                    g.set_param(cap.id, p.id, max * 0.6);
+            for (i, p) in cap.params.iter().enumerate() {
+                if let dr_pipeline::ParamKind::Scalar { min, max, .. } = p.kind {
+                    // Stepped away from each parameter's own default by a
+                    // varying fraction. A single shared value would leave the
+                    // tone curve inactive: its neutral is the *relationship*
+                    // between its points, so setting them all alike keeps it
+                    // on the identity diagonal.
+                    let step = (max - min) * (0.15 + 0.05 * (i % 4) as f32);
+                    let v = if p.default + step <= max {
+                        p.default + step
+                    } else {
+                        p.default - step
+                    };
+                    g.set_param(cap.id, p.id, v);
                 }
             }
         }