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/ops/mod.rs

@@ -15,6 +15,7 @@ pub mod aberration;
 pub mod colour;
 pub mod colour_mixer;
 pub mod contrast;
+pub mod curve;
 pub mod distortion;
 pub mod exposure;
 pub mod helpers;
@@ -26,6 +27,7 @@ pub use aberration::Aberration;
 pub use colour::{Brilliance, Saturation, Vibrance};
 pub use colour_mixer::ColourMixer;
 pub use contrast::Contrast;
+pub use curve::ToneCurve;
 pub use distortion::Distortion;
 pub use exposure::Exposure;
 pub use tone::{BlacksWhites, HighlightsShadows};