fix: sidebar refresh, context menu position, show-in-explorer for all items

Sidebar refresh:
- addFile/addFolder now use currentFolderId || selectedNode.id as parent
- startImport stores pendingImportParent; confirmImport uses it
- setNodeChildren also updates node.has_children for toggle arrow reactivity
- navigateToFolder expands the folder node in sidebar tree

Context menu position:
- FileTreeRow stores menuX/menuY from contextmenu event coords
- Menu uses position:fixed with cursor-relative left/top and window clamp

Show in explorer:
- Sidebar context menu uses file.showInExplorer label
- en.js: add file.showInExplorer key

frontend/src/lib/i18n/locales/en.js

@@ -74,6 +74,7 @@ export default {
   'file.preview': 'Preview',
   'file.openExternal': 'Open in external program',
   'file.openFolder': 'Open folder',
+  'file.showInExplorer': 'Show in explorer',
   'file.delete': 'Delete',
   'file.pickSingle': 'Select file',
   'file.pickDirectory': 'Select folder',