feat(move): "move to folder" for grid selections, folders, and m shortcut

Extends the heap-only "move to folder" action to grid single/bulk
selections, sidebar folders, and an `m` keyboard shortcut — all through
one shared dialog driven by a moveDialog store.

Backend (sidecar):
- Extract the heap move/copy + reindex loop into a reusable movePhotoFiles
  helper plus resolveMoveTarget
- POST /photos/move: move/copy an arbitrary UID list into a folder
- POST /folders/:rel/move: reparent a folder dir (whole subtree) under a
  new parent, guarding against moving into itself/a descendant

Frontend:
- moveDialog store + generalized MoveToFolderDialog (heap | photos | folder
  subjects); mounted once in +layout.svelte. Replaces HeapConvertDialog
- movePhotosToFolder / moveFolder service fns
- Entry points: BulkActionBar button, gridKeyNav `m`, FolderTree kebab,
  heap kebab — all call openMove()

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/routes/+layout.svelte

@@ -19,6 +19,7 @@
 	import LeftSidebar from '$lib/components/layout/LeftSidebar.svelte';
 	import AnimatedMule from '$lib/components/mule/AnimatedMule.svelte';
 	import PreviewModal from '$lib/components/preview/PreviewModal.svelte';
+	import MoveToFolderDialog from '$lib/components/layout/MoveToFolderDialog.svelte';
 
 	let { children } = $props();
 
@@ -117,6 +118,10 @@
 		     helper (called by the timeline / PhotoGrid dblclick paths and
 		     by gridKeyNav's Space handler). -->
 		<PreviewModal />
+		<!-- Single shared move-to-folder dialog, driven by the moveDialog
+		     store. Opened from the heap/folder kebabs, the BulkActionBar
+		     button, and the `m` shortcut — all through openMove(). -->
+		<MoveToFolderDialog />
 	{:else}
 		{@render children?.()}
 	{/if}