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/lib/components/timeline/BulkActionBar.svelte

@@ -26,6 +26,7 @@
 	import { filters } from '$lib/stores/filters.svelte';
 	import { push as pushUndo } from '$lib/stores/undo.svelte';
 	import { isAuthenticated } from '$lib/stores/session.svelte';
+	import { openMove } from '$lib/stores/moveDialog.svelte';
 	import {
 		startBulk,
 		setDetail,
@@ -455,6 +456,15 @@
 					Archive
 					<kbd class="rounded bg-muted px-1 text-[9px] font-medium text-muted-foreground">X</kbd>
 				</button>
+				<button
+					class="inline-flex items-center gap-1 rounded border border-border bg-background px-2 py-0.5 text-[11px] hover:bg-accent disabled:opacity-50"
+					disabled={busy}
+					onclick={() => openMove({ kind: 'photos', uids: snapshotIds() })}
+					title="Move selected photos to a folder"
+				>
+					Move to folder
+					<kbd class="rounded bg-muted px-1 text-[9px] font-medium text-muted-foreground">M</kbd>
+				</button>
 			{/if}
 			<button
 				class="inline-flex items-center gap-1 rounded px-2 py-0.5 text-[11px] text-muted-foreground hover:bg-accent hover:text-foreground"