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/ui/app.slint

@@ -1,5 +1,6 @@
 import { Theme } from "theme.slint";
 import { AdjustPanel, ParamRow } from "adjust.slint";
+import { LaunchScreen } from "launch.slint";
 
 // Status strip — surfaces the GPU backend and adapter, which matters during
 // v0.1 because assumption A1 is exactly "does this compositing path work on
@@ -137,6 +138,30 @@ export component AppWindow inherits Window {
     in property <int> total: 0;
     in property <string> load-error: "";
 
+    // --- launch screen (FR-NC-1, FR-NC-4) ---
+    //
+    // The app opens here when no library is configured, and returns here to
+    // sign out or switch account.
+    in property <bool> show-launch: false;
+    in property <bool> launch-signed-in: false;
+    in property <string> launch-account: "";
+    in property <string> launch-root: "";
+    in property <string> launch-server: "";
+    in property <bool> launch-busy: false;
+    in property <string> launch-status: "";
+    in property <string> launch-error: "";
+    in property <string> launch-login-url: "";
+    in property <bool> launch-can-remember: true;
+    in-out property <[string]> launch-format-labels;
+    in-out property <[bool]> launch-format-checked;
+
+    callback launch-sign-in(string);
+    callback launch-sign-out();
+    callback launch-choose-folder();
+    callback launch-open-library();
+    callback launch-format-toggled(int, bool);
+    callback launch-copy-url();
+
     callback next-image();
     callback prev-image();
 
@@ -164,7 +189,32 @@ export component AppWindow inherits Window {
     // One-way: report width outward, never read layout back into it.
     changed width => { root.window-resized(self.width); }
 
-    VerticalLayout {
+    // The launch screen replaces the whole window rather than overlaying it:
+    // there is no library to look at until an account is configured.
+    if root.show-launch: LaunchScreen {
+        width: 100%;
+        height: 100%;
+        signed-in: root.launch-signed-in;
+        account: root.launch-account;
+        library-root: root.launch-root;
+        server-url: root.launch-server;
+        busy: root.launch-busy;
+        status: root.launch-status;
+        error: root.launch-error;
+        login-url: root.launch-login-url;
+        can-remember: root.launch-can-remember;
+        format-labels: root.launch-format-labels;
+        format-checked: root.launch-format-checked;
+
+        sign-in(server) => { root.launch-sign-in(server); }
+        sign-out() => { root.launch-sign-out(); }
+        choose-folder() => { root.launch-choose-folder(); }
+        open-library() => { root.launch-open-library(); }
+        format-toggled(i, on) => { root.launch-format-toggled(i, on); }
+        copy-login-url() => { root.launch-copy-url(); }
+    }
+
+    if !root.show-launch: VerticalLayout {
         StatusBar {
             adapter: root.adapter;
             backend: root.backend;