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/operation.rs

@@ -22,7 +22,7 @@
 
 use std::fmt::Write as _;
 
-use crate::descriptor::{OpDescriptor, ParamId};
+use crate::descriptor::{OpDescriptor, ParamId, Presentation};
 use crate::framing::{Framing, FRAMING_UNIFORM_FIELDS};
 
 /// What an operation's parameters affect, for cache invalidation scoping.
@@ -100,6 +100,22 @@ pub trait Operation: Send + Sync {
     fn helpers(&self) -> &'static [Helper] {
         &[]
     }
+
+    /// TRACES: FR-DEV-3a | FR-DEV-3b
+    /// How this operation would like its parameters presented.
+    ///
+    /// `None` — the default, and the right answer for nearly every operation
+    /// — means one control per parameter, chosen from its
+    /// [`crate::ParamKind`]. Returning a [`Presentation`] says that several
+    /// parameters form a single conceptual control and names the widget that
+    /// draws it.
+    ///
+    /// Purely a hint. The parameters remain individually addressable
+    /// scalars, so a UI that does not implement the named widget falls back
+    /// to sliders and stays fully functional.
+    fn presentation(&self) -> Option<Presentation> {
+        None
+    }
 }
 
 /// A named WGSL helper function, deduplicated across operations.