Give the drag a pick-up, so it stops losing to the scroll

Dragging a photograph out of the grid worked about half the time, and
nothing on the screen explained the other half.

`DragArea` and `Flickable` do arbitrate, but not evenly. The Flickable
claims any press that travels more than eight pixels along its own axis
within half a second of landing, and holds that claim until the finger
lifts. So a drag toward the sidebar only ever began two ways: a flick
sideways clean enough that the finger never wandered eight pixels
vertically, or a wait of half a second before moving at all. Both are
real gestures and neither was written down.

The wait is now the gesture, and it has a mark. The long press that
already turns on selection mode also picks the photograph up: a ring
opens around the cell and the grid stops scrolling under it, so from
that moment the drag is the only thing the finger can be doing. The cue
can only arrive after the ambiguity has passed, which is the right way
round — when the photograph lifts, dragging it works.

Two details worth naming. The hold is now armed even when selection mode
is already on; it used to be skipped there, on the grounds that there
was no mode left to switch on — but that is precisely the state a
forty-image drag starts from, so the one gesture that most needed a
pick-up was the one with none. And the ring is drawn after the cell
loop rather than on the cell: z-order inside a `for` is loop order, so a
cell grown past its bounds would stand over two neighbours and be cut
off by the other two.

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

ui/dr-ui/ui/app.slint

@@ -543,6 +543,13 @@ export component AppWindow inherits Window {
     /// that turns it on. The long press does the same thing without it.
     in property <bool> library-select-mode: false;
     callback library-toggle-select-mode();
+    /// TRACES: FR-CAT-7 | FR-UI-4
+    /// The row a press has held long enough to pick up, or `-1`.
+    ///
+    /// Set by the same hold that turns on selection mode and cleared by the
+    /// grid when the press ends — `in-out` because both ends write it. See
+    /// `held-row` in `library.slint` for what it draws and what it stops.
+    in-out property <int> library-held-row: -1;
     /// A press on a cell ended, so the long-press timer can be cancelled.
     callback library-cell-press-ended();
     /// TRACES: FR-UI-2 | FR-UI-4
@@ -1579,6 +1586,7 @@ in property <bool> panel-visible: true;
                     cell-press-ended() => { root.library-cell-press-ended(); }
                     select-mode: root.library-select-mode;
                     toggle-select-mode() => { root.library-toggle-select-mode(); }
+                    held-row <=> root.library-held-row;
                     // The sidebar's rows, not a second model: the sheet files
                     // into the same tree the sidebar draws.
                     collections: root.collection-rows;