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/layout/FolderTree.svelte

@@ -41,7 +41,7 @@
 	import { filters } from '$lib/stores/filters.svelte';
 	import { browser } from '$app/environment';
 	import { untrack } from 'svelte';
-	import { FolderPlus, Pencil, Trash2 } from 'lucide-svelte';
+	import { FolderInput, FolderPlus, Pencil, Trash2 } from 'lucide-svelte';
 	import Self from './FolderTree.svelte';
 	import KebabMenu, { Item, Separator } from './KebabMenu.svelte';
 
@@ -50,10 +50,13 @@
 		depth?: number;
 		onPick: (path: string) => void;
 		/** Mutating callbacks are only required when readonly !== true. The
-		 *  picker (HeapConvertDialog) reuses the tree just for `onPick`. */
+		 *  picker (MoveToFolderDialog) reuses the tree just for `onPick`. */
 		onRename?: (path: string) => void;
 		onDelete?: (path: string) => void;
 		onCreateChild?: (parent: string) => void;
+		/** Reparent this folder under a chosen destination (opens the shared
+		 *  move-to-folder dialog). Sidebar only; the readonly picker omits it. */
+		onMove?: (path: string) => void;
 		/** Read-only mode: hides the kebab menu and disables double-click
 		 *  rename, so the tree can be reused as a folder picker. */
 		readonly?: boolean;
@@ -74,6 +77,7 @@
 		onRename,
 		onDelete,
 		onCreateChild,
+		onMove,
 		readonly = false,
 		selectedPath,
 		counts
@@ -219,6 +223,13 @@
 								<Pencil class="h-3.5 w-3.5 text-muted-foreground" />
 								Rename
 							</Item>
+							<Item
+								class="flex cursor-pointer items-center gap-2 rounded px-2 py-1.5 text-[12px] outline-none hover:bg-accent focus:bg-accent"
+								onSelect={() => onMove?.(node.path)}
+							>
+								<FolderInput class="h-3.5 w-3.5 text-muted-foreground" />
+								Move to folder…
+							</Item>
 							<Separator class="my-1 h-px bg-border" />
 							<Item
 								class="flex cursor-pointer items-center gap-2 rounded px-2 py-1.5 text-[12px] text-destructive outline-none hover:bg-destructive/10 focus:bg-destructive/10"
@@ -239,6 +250,7 @@
 					{onRename}
 					{onDelete}
 					{onCreateChild}
+					{onMove}
 					{readonly}
 					{selectedPath}
 					{counts}