Make a pinch zoom the grid and nothing else

Two faults left over from making the gesture reach the grid at all.

**It still opened photographs.** Checking the finger id stops the
synthetic release Slint emits when the *second* finger lands, but not the
other end of the gesture: lifting one finger of two leaves the other one
down, and Slint replays that survivor as a fresh `Pressed` on whatever is
under it — which is how it hands the pointer back to ordinary handling.
Under it is a cell. So the cell was selected, and lifting that last finger
was a complete, well-formed click on the same finger that pressed. No
part of the event stream distinguishes it from a real tap, so the grid now
remembers that a pinch just happened: a latch raised when the gesture
starts and lowered a beat after it ends, during which cells take neither
presses nor clicks.

The press that *opens* a pinch is undone rather than suppressed — it has
already happened by the time a second finger makes it a pinch. Undoing it
has to be exact, or a cancel that restored the selection but left the
anchor moved would make the next shift-click select a run from a cell
nobody pointed at, so capture and restore are a tested pair.

**And it was not smooth.** Two reasons. The pinch was thresholded into ±1
steps of 25%, so the grid lurched and then sat still; it now takes the
ratio since the last update and tracks the fingers, with the drawn cell
still landing on whole column counts because the columns divide the
width. And `zoom-cells` was the one geometry change still reloading
inline — a full catalog re-read, 360-row model rebuild and thumbnail
batch per step, on the thread drawing the frame. It goes through the same
settle timer as the rest now.

ui/dr-ui/ui/app.slint

@@ -508,6 +508,10 @@ export component AppWindow inherits Window {
     in property <bool> library-syncing: false;
     in-out property <length> library-cell-size: 180px;
     callback library-zoom-cells(int);
+    /// A pinch step on the grid: the ratio since the last update.
+    callback library-pinch-cells(float);
+    /// A pinch on the grid has begun, so the press that opened it was not one.
+    callback library-pinch-started();
     in property <int> library-scroll-to: 0;
     in property <int> library-scroll-token: 0;
     /// Which row of the loaded window is the photograph currently open in
@@ -1130,6 +1134,8 @@ in property <bool> panel-visible: true;
             syncing: root.library-syncing;
             requested-cell-size: root.library-cell-size;
             zoom-cells(d) => { root.library-zoom-cells(d); }
+            pinch-cells(r) => { root.library-pinch-cells(r); }
+            pinch-started() => { root.library-pinch-started(); }
             scroll-to: root.library-scroll-to;
             scroll-token: root.library-scroll-token;
             sync-now() => { root.library-sync-now(); }