picker: Fix layout persistency with preview setting change (#59827)

## Summary

When you change a picker's preview position (toggle it, or set it
right/below/hidden), that choice should be remembered the next time the
picker opens. Previously it wasn't: the last-used layout was only
written to the database as a side effect of
`store_shape_for_this_layout`, which runs exclusively when a resize
completes (`render.rs`). So changing only the preview position without
also dragging to resize, was forgotten on the next open.

All layout changes funnel through a single chokepoint,
`set_preview_layout`, but it only updated the in-memory `preview.layout`
and reloaded the per-layout shape; it never persisted the layout key.

## Solution

`set_preview_layout` now persists the layout immediately via a new
`store_last_layout` helper, which writes just the
`LAST_PREVIEW_LAYOUT` key — deliberately without writing the current
shape, so a never-resized layout's default size isn't frozen into the
database as if the user had chosen it.

  Both persistence paths are now correct:
  - Resize completes — saves size and layout (unchanged).
- Position-only change (toggle / set right / set below / set hidden)
saves the layout right away.

This applies to every picker with a preview (file finder, text finder),
since the persistence is keyed generically by the picker delegate name.

## Testing

- Manual: open the file and text finder → switch the preview to Below
(or Right) without resizing → close and reopen → it returns in that
position; hide the preview → reopen → it stays hidden. Resizing still
persists size as before.

Release Notes:

- Fixed picker preview position not being remembered when changed
without resizing the picker

---------

Co-authored-by: ozacod <ozacod@users.noreply.github.com>

crates/picker/src/persistence.rs

@@ -36,6 +36,26 @@ pub(crate) fn store_shape_for_this_layout(
     });
 }
 
+/// Persist the last-used preview layout without touching the stored shape.
+///
+/// Changing the preview position (e.g. right/below/hidden) without resizing
+/// must still be remembered, so this is called whenever the layout changes.
+pub(crate) fn store_last_layout(
+    picker_delegate: &'static str,
+    preview_layout: Option<preview::Layout>,
+    cx: &App,
+) {
+    let kvp = KeyValueStore::global(cx);
+    db::write_and_log(cx, async move || {
+        kvp.scoped(PICKERS_NAMESPACE)
+            .write(
+                last_layout_key(picker_delegate),
+                layout_as_str(preview_layout).to_string(),
+            )
+            .await
+    });
+}
+
 pub(crate) fn try_load_shape(
     picker_delegate: &'static str,
     preview_layout: impl Into<Option<preview::Layout>>,

crates/picker/src/picker.rs

@@ -1260,6 +1260,8 @@ impl<D: PickerDelegate> Picker<D> {
         _window: &mut Window,
         cx: &mut Context<Self>,
     ) {
+        persistence::store_last_layout(D::name(), self.preview.as_ref().map(|_| layout), cx);
+
         let Some(preview) = &mut self.preview else {
             return;
         };