Let the launch screen scroll, now that it offers three routes

The signed-out screen was a `VerticalLayout { alignment: center }` with
no scroll. That was already tight with two routes; the folder option
made the column taller than a 900x560 window, and a centred layout that
overflows clips at *both* ends — so the masthead and the last route
disappear together, with nothing on screen to suggest either existed.

A Flickable whose viewport follows the content, and a top padding that
centres the column only while it fits. Both cases checked against the
running app: centred at 1000x1800, top-aligned and scrollable at
900x560.

ui/dr-ui/ui/launch.slint

@@ -93,15 +93,37 @@ export component LaunchScreen inherits Rectangle {
         init => { self.focus(); }
     }
 
-    VerticalLayout {
+    // Scrollable, and it has to be. The signed-out screen offers three routes
-        alignment: center;
+    // — browser sign-in, an app password, a folder — and the column is taller
-        padding: Theme.gap-lg;
+    // than a laptop window in a side-by-side split, let alone a phone. A
+    // centred `VerticalLayout` that overflows clips at *both* ends, so the
+    // masthead and the last route disappear together and there is no
+    // indication either existed.
+    //
+    // `viewport-height` follows the content rather than being fixed: the
+    // screen's height changes by hundreds of pixels as it moves between
+    // signed-out, awaiting approval, and signed-in, and a constant would
+    // either strand a scrollbar on the short states or clip the tall one.
+    Flickable {
+        viewport-height: max(self.height, column.preferred-height);
+
+        column := VerticalLayout {
+            width: 100%;
+            alignment: start;
+            padding: Theme.gap-lg;
+            // Centres the column when it fits and lets it start at the top
+            // when it does not — the two cases the fixed `alignment: center`
+            // could not both serve.
+            padding-top: max(
+                Theme.gap-lg,
+                (root.height - card.preferred-height - 2 * Theme.gap-lg) / 2
+            );
 
         Rectangle {
             max-width: 460px;
             horizontal-stretch: 0;
 
-            VerticalLayout {
+            card := VerticalLayout {
                 spacing: Theme.gap-lg;
 
                 // --- masthead ---
@@ -423,5 +445,6 @@ export component LaunchScreen inherits Rectangle {
                 }
             }
         }
+        }
     }
 }