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>

sidecar/main.go

@@ -91,9 +91,11 @@ func main() {
 			auth.POST("/folders", handleFolderCreate(cfg, pp))
 			auth.POST("/folders/counts", handleFolderCounts(pp))
 			auth.POST("/folders/:rel/rename", handleFolderRename(cfg, pp))
+			auth.POST("/folders/:rel/move", handleFolderMove(cfg, pp))
 			auth.DELETE("/folders/:rel", handleFolderDelete(cfg, pp))
 
 			auth.POST("/albums/:uid/convert", handleHeapConvert(cfg, pp))
+			auth.POST("/photos/move", handlePhotosMove(cfg, pp))
 
 			auth.GET("/duplicates/scan", handleDupScan(cfg, pp))
 			auth.POST("/duplicates/archive", handleDupArchive(cfg, pp))