Add folders for organizing saved conversations

Conversations can now be filed into flat (non-nested) folders, shown
as collapsible sections in both the sidebar and the advanced
conversation list. New folders migration (v9) adds a folders table
and conversations.folderId with ON DELETE SET NULL, so deleting a
folder unfiles its conversations rather than losing them. Move/rename/
delete via context menu; conversation lists with no folders fall back
to the existing flat view unchanged.

oAI/Models/Folder.swift

@@ -0,0 +1,43 @@
+//
+//  Folder.swift
+//  oAI
+//
+//  Model for grouping saved conversations
+//
+// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0
+// Copyright (C) 2026 Rune Olsen
+//
+// This file is part of oAI.
+//
+// oAI is licensed under the PolyForm Noncommercial License 1.0.0.
+// You may use, study, modify, and share it for any noncommercial
+// purpose. Commercial use — including selling oAI or any part of
+// it, standalone or bundled into another product or service —
+// requires a separate commercial license from the copyright holder.
+//
+// See the LICENSE file or
+// <https://polyformproject.org/licenses/noncommercial/1.0.0> for
+// the full license text. For commercial licensing, contact Rune
+// Olsen via <https://oai.pm>.
+
+
+import Foundation
+
+struct Folder: Identifiable, Codable, Sendable {
+    let id: UUID
+    var name: String
+    var sortOrder: Int
+    let createdAt: Date
+
+    nonisolated init(
+        id: UUID = UUID(),
+        name: String,
+        sortOrder: Int = 0,
+        createdAt: Date = Date()
+    ) {
+        self.id = id
+        self.name = name
+        self.sortOrder = sortOrder
+        self.createdAt = createdAt
+    }
+}