Ask for the collection's name where the keyboard can reach it

"New collection from selection" created the collection under a placeholder
name and then opened the rename field in the sidebar tree.

On a tablet the sidebar is not on screen. It is instantiated all the same —
app.slint collapses it to zero width and `visible: false` rather than using an
`if`, because an `if` there is a layout loop Slint panics on — so the rename
field was created, its `init` took focus, and Android raised the on-screen
keyboard over a box nobody could see. Nothing else on the screen is focusable,
so the keyboard had nowhere to go: it stayed, the name could not be typed, and
the collection was already written under the name the user did not want.

Asked in a sheet instead, on the same card as the filing and keywording sheets,
before anything is written. That also fixes what was hiding behind it: an
abandoned rename used to leave a "New collection" in the tree, because the
collection existed before the name did.

`Field` gains `take-focus()` so a sheet whose field is the only thing to do in
it can answer the keyboard for the user — a function rather than a property,
because focus is an event and a bound property would re-take it on every
unrelated re-evaluation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/ui/app.slint

@@ -291,7 +291,7 @@ export component AppWindow inherits Window {
     callback library-toggle-date-range();
     /// TRACES: FR-CAT-5
     callback library-clear-selection();
-    callback library-collection-from-selection();
+    callback library-collection-from-selection(string);
     /// TRACES: FR-CAT-6
     in property <string> library-range-from;
     in property <string> library-range-to;
@@ -1380,7 +1380,9 @@ in property <bool> panel-visible: true;
                     range-active: root.library-range-active;
                     toggle-date-range() => { root.library-toggle-date-range(); }
                     clear-selection() => { root.library-clear-selection(); }
-                    collection-from-selection() => { root.library-collection-from-selection(); }
+                    collection-from-selection(name) => {
+                        root.library-collection-from-selection(name);
+                    }
                     range-from: root.library-range-from;
                     range-to: root.library-range-to;
                     range-invalid: root.library-range-invalid;