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.

ui/dr-ui/Cargo.toml

@@ -12,7 +12,11 @@ dr-types.workspace = true
 # must come off when S1 lands (ARCH §6.1, AC-8).
 dr-gpu = { workspace = true, features = ["readback"] }
 dr-decode.workspace = true
+serde_json.workspace = true
+tokio.workspace = true
+reqwest.workspace = true
 dr-plat.workspace = true
+dr-sync.workspace = true
 dr-sync-nextcloud.workspace = true
 dr-pipeline.workspace = true
 slint = { workspace = true, features = ["compat-1-2", "renderer-femtovg", "backend-winit"] }