Let a collection be picked up, rearranged, and emptied after the fact

Collections could be made and filled and never reorganised. Nesting had
a drag; un-nesting had nothing, in either direction — "All photographs"
refused every drop, which is right for a photograph and wrong for a
collection, which has a top level to be returned to. So a collection put
inside another was in there permanently. Right-click deleted an *empty*
collection outright and refused otherwise, which is wrong in both
directions at once: destructive with no confirmation, and no way at all
to delete a collection that held anything without emptying it by hand,
child by child. And a photograph could only leave the collection the
grid was scoped to, since that is the only one a button in the header
can name — the cell's badge says a photograph is in three collections
and never which three.

Three ways in, one vocabulary:

**Hold a row.** The tree is inside a Flickable, which claims any drag
beginning inside it, so with a finger a drag on a row is a scroll until
something says otherwise. The hold is that something. It lifts the row —
drawn before anything moves, so the gesture says it has been understood
— and then what the user does decides which of two things they meant:
move, and it is a rearrangement; let go, and it is the row menu. The
same fork the grid already uses to tell hold-to-select from drag-to-file.
`decide_release` is that fork, and it is tested, because getting it
wrong one way puts a sheet over every tidied tree and the other way
makes the menu unreachable by touch.

**The row menu.** Rename, new collection inside, move to top level,
keep offline, delete. Deleting asks once when there is anything to lose
and says what survives: the photographs stay in the library, and nested
collections move up rather than going with it — which is what the
catalog does, and what a user would never assume. An empty collection
goes on the first press, because a dialogue about losing nothing is how
people learn to dismiss dialogues.

**"Collections…" on a selection.** Every collection the selection is
filed in, each with a count — "3 of 40", so nobody takes forty
photographs out of a collection thirty-seven were never in — and a way
out of any of them without navigating there first.

The long press used to open the offline question by itself. That
question is one item in this menu now: there is one hold per row, and
while it was spent on a single action nothing else the tree can do had
a touch route at all. Nothing is lost — the tray on the row keeps its
tap, and the question gains a full-width control in place of a 30px
icon in a row shorter than the touch minimum.

The row-press handler moves to `collections_ui` with the rest of what a
collection row does; it lived in `library_ui` only because it opened
that prompt.

ui/dr-ui/ui/library.slint

@@ -1617,6 +1617,12 @@ export component LibraryGrid inherits Rectangle {
     /// offered when the grid is scoped to one — "remove from library" is not a
     /// thing this button does.
     callback remove-from-collection();
+    /// Show every collection the selection is filed in, with a way out of each.
+    /// The button above can only ever name the scoped collection; this is how a
+    /// photograph leaves one the grid is not currently showing — and the only
+    /// way to find out which collections it is in at all, since the cell badges
+    /// a count and nothing else.
+    callback open-membership();
 
     // --- filing the selection (FR-CAT-7, FR-UI-4) ---------------------------
     //
@@ -4248,6 +4254,27 @@ export component LibraryGrid inherits Rectangle {
                 clicked => { root.remove-from-collection(); }
             }
 
+            // TRACES: FR-CAT-7 | FR-UI-4
+            // Where this selection is filed, and the way out of any of it.
+            //
+            // Unscoped as well as scoped, unlike the button above: this is the
+            // one control that can name a collection the grid is not showing,
+            // and hiding it outside a scope would leave "which collections is
+            // this photograph in" answerable only by visiting each one.
+            // GESTURE: Take photographs out of a collection
+            //   where:   Library grid
+            //   touch:   Select them, then "Collections…" in the selection bar
+            //   pointer: Select them, then "Collections…" in the selection bar
+            //   why:     The badge on a cell says a photograph is filed in
+            //            three collections and never which. This is the sheet
+            //            that names them, and the only way out of one the grid
+            //            is not currently scoped to.
+            if !root.ranging && root.selected-count > 0: Button {
+                text: "Collections…";
+                y: (parent.height - self.height) / 2;
+                clicked => { root.open-membership(); }
+            }
+
             // TRACES: FR-EXP-7 | NFR-ARCH-3
             // Export the selection, and stop the batch that is running.
             //