Sort folders alphabetically; let users pick the merge model directly

Folders now list alphabetically (case-insensitive) everywhere they're
shown — sidebar, advanced conversation list, and the "Move to Folder"
menu — rather than creation order. listFolders() does the sort at the
DB layer; in-memory folder arrays are re-sorted after local
create/rename so newly added or renamed folders don't fall out of
order until the next reload.

Combine Conversations' AI-Assisted Merge no longer silently assumes
settings.defaultModel — added a model picker (reusing ModelSelectorView,
defaulting to the current default model/provider) so users can pick
which model performs the merge per-combine. ConversationMergeService.merge
takes optional mergeModelId/mergeProvider overrides.

oAI/Views/Main/SidebarView.swift

@@ -357,6 +357,7 @@ struct SidebarView: View {
         do {
             let folder = try DatabaseService.shared.createFolder(name: name)
             folders.append(folder)
+            sortFolders()
             if let conversation = conversation {
                 moveConversation(conversation, toFolder: folder.id)
             }
@@ -366,6 +367,10 @@ struct SidebarView: View {
         #endif
     }
 
+    private func sortFolders() {
+        folders.sort { $0.name.localizedCaseInsensitiveCompare($1.name) == .orderedAscending }
+    }
+
     private func renameFolderPrompt(_ folder: Folder) {
         #if os(macOS)
         let alert = NSAlert()
@@ -385,6 +390,7 @@ struct SidebarView: View {
             if let i = folders.firstIndex(where: { $0.id == folder.id }) {
                 folders[i].name = newName
             }
+            sortFolders()
         } catch {
             Log.db.error("Failed to rename folder: \(error.localizedDescription)")
         }