Add nested (hierarchical) conversation folders

Folders can now contain other folders, arbitrarily deep — e.g. "Work"
containing "Project A"/"Project B". v10 migration adds a
self-referencing parentId column; tree ordering, depth, and cycle
detection are pure Swift (Folder.orderedTree/isDescendant/
visibleFolderIds), not SQL, so listFolders() stays a simple flat
query.

- Create nested folders via "New Subfolder…" (context menu, both
  list views) or by dragging a folder onto another to reparent it.
  Dragging onto an existing descendant is rejected (cycle guard).
- Deleting a folder reparents its children and any conversations
  filed directly in it up one level to the deleted folder's own
  parent — conversations are never deleted. This also fixes a real
  bug: the previous deleteFolder never persisted unfiling to the
  database, only patched in-memory state, so a conversation whose
  folder was deleted kept a dangling folderId and silently vanished
  from view after the next relaunch.
- All "Move to Folder" pickers (sidebar, advanced list, per-row
  context menus, the Save dialog's folder popup) show an indented
  flat list reflecting the tree.
- New DraggedItem enum disambiguates a dragged folder from dragged
  conversation(s) in the shared string-based drag payload, and
  unifies both list views on the same bundled-multi-selection format
  — closes a gap where dragging a multi-selection in the advanced
  list (⌘L) only moved the one row grabbed, unlike the sidebar.

Confirmed working live, including relaunch-survival of the
delete/reparent fix.

oAI/ViewModels/ChatViewModel.swift

@@ -35,6 +35,7 @@ private final class ConversationSaveAccessory: NSObject {
     let nameField: NSTextField
     private let folderPopup: NSPopUpButton
     private var folders: [Folder]
+    private var entries: [(folder: Folder, depth: Int)] = []  // recomputed in rebuildMenu whenever folders changes
     private var lastGoodSelection: UUID?  // the folder to revert to if "New Folder…" is cancelled
 
     init(defaultName: String, folders: [Folder], selectedFolderId: UUID?) {
@@ -56,15 +57,16 @@ private final class ConversationSaveAccessory: NSObject {
     }
 
     private func rebuildMenu(selecting folderId: UUID?) {
+        entries = Folder.orderedTree(from: folders)
         folderPopup.removeAllItems()
         folderPopup.addItem(withTitle: "No Folder")
-        for folder in folders {
-            folderPopup.addItem(withTitle: folder.name)
+        for entry in entries {
+            folderPopup.addItem(withTitle: String(repeating: "  ", count: entry.depth) + entry.folder.name)
         }
         folderPopup.menu?.addItem(.separator())
         folderPopup.addItem(withTitle: "New Folder…")
 
-        if let folderId, let idx = folders.firstIndex(where: { $0.id == folderId }) {
+        if let folderId, let idx = entries.firstIndex(where: { $0.folder.id == folderId }) {
             folderPopup.selectItem(at: idx + 1)
         } else {
             folderPopup.selectItem(at: 0)
@@ -104,8 +106,8 @@ private final class ConversationSaveAccessory: NSObject {
 
     var resolvedFolderId: UUID? {
         let idx = folderPopup.indexOfSelectedItem
-        guard idx >= 1, idx - 1 < folders.count else { return nil }
-        return folders[idx - 1].id
+        guard idx >= 1, idx - 1 < entries.count else { return nil }
+        return entries[idx - 1].folder.id
     }
 }
 #endif