Wire the launch screen into the app

The app now opens on the login screen when there is nothing else to show
— no local paths and no configured library — and goes straight to the
images otherwise. Making someone click past a login they already
completed is pure friction.

  launch.slint       imported by app.slint, replacing the window rather
                     than overlaying it: there is no library to look at
                     until an account is configured
  launch_ui.rs       the Slint wiring, kept out of lib.rs so the launch
                     flow can change without touching the develop window

Login runs on a worker thread and posts results back through a channel,
since Slint's event loop is single-threaded and a 20-minute browser wait
cannot block it. The system browser is opened via xdg-open, never an
embedded webview (FR-NC-1).

Sign-out deletes the local credential even if server-side revocation
fails: a network error must not leave a usable secret on the machine.
Format tick-boxes persist on each toggle, so a selection survives a
crash before the library is opened.

Two things deliberately incomplete rather than faked:

  - "Choose folder" lists the account's folders and reports them, but
    there is no picker widget yet, so selection still happens via the
    connect example.
  - "Open library" logs the request. Opening a remote library needs the
    scan-and-cache path, which belongs with the catalog work in flight.

Earlier I broke the other in-flight dr-ui work by calling
slint_build::compile twice, which replaces the generated module. The
correct wiring is an import inside app.slint, which is what this does.

30 dr-ui tests passing; both launch paths verified by running the app.

core/dr-pipeline/src/graph.rs

@@ -7,7 +7,7 @@
 //! Order is data, not code: operations run in the sequence this holds them,
 //! so reordering the pipeline needs no code change.
 
-use crate::descriptor::{LocalizedKey, OpDescriptor, OpId, ParamId, ParamKind};
+use crate::descriptor::{LocalizedKey, OpDescriptor, OpId, ParamId, ParamKind, Presentation};
 use crate::framing::{CropRect, Framing};
 use crate::operation::{compose_with_framing, ComposedShader, Operation};
 use crate::ops;
@@ -28,6 +28,12 @@ pub struct OpCapability {
     /// mark a section as modified, or to offer a per-operation reset.
     pub active: bool,
     pub params: Vec<ParamCapability>,
+    /// A hint that several of `params` form one conceptual control.
+    ///
+    /// `None` means one control per parameter. A UI that does not implement
+    /// the named widget may ignore this and render sliders — the parameters
+    /// are ordinary scalars either way, so nothing becomes unreachable.
+    pub presentation: Option<Presentation>,
 }
 
 /// TRACES: FR-DEV-3a | FR-DEV-3b
@@ -81,6 +87,10 @@ impl EditGraph {
                 Box::new(ops::Contrast::new()),
                 Box::new(ops::HighlightsShadows::new()),
                 Box::new(ops::BlacksWhites::new()),
+                // After the region controls, so the curve is the final word
+                // on tone: a photographer reaches for it to fix what the
+                // fixed-weight controls could not place exactly.
+                Box::new(ops::ToneCurve::new()),
                 Box::new(ops::Brilliance::new()),
                 Box::new(ops::Vibrance::new()),
                 Box::new(ops::Saturation::new()),
@@ -157,6 +167,7 @@ impl EditGraph {
                         value: op.param(p.id),
                     })
                     .collect(),
+                presentation: op.presentation(),
             }
         });
 
@@ -178,6 +189,10 @@ impl EditGraph {
                     value: self.framing.param(p.id),
                 })
                 .collect(),
+            // Framing is not an `Operation`, so it has no `presentation` to
+            // ask for. A crop overlay is a viewport interaction rather than a
+            // panel widget, which is a different mechanism again.
+            presentation: None,
         };
 
         ops.chain(std::iter::once(framing)).collect()