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.

oAITests/DatabaseServiceTests.swift

@@ -220,3 +220,96 @@ struct DatabaseServiceUsageStatsTests {
         #expect(byConversation.count == 3)
     }
 }
+
+@Suite("DatabaseService folders, against a throwaway in-memory queue")
+struct DatabaseServiceFolderTests {
+
+    @Test("v9 adds the folders table and folderId to conversations")
+    func v9AddsFolderSupport() {
+        let db = DatabaseService.makeInMemory()
+        #expect(db.tableExists("folders"))
+        #expect(db.columnNames(in: "conversations").contains("folderId"))
+    }
+
+    @Test("Creating folders assigns increasing sort order")
+    func createFolderAssignsSortOrder() throws {
+        let db = DatabaseService.makeInMemory()
+        let first = try db.createFolder(name: "Work")
+        let second = try db.createFolder(name: "Personal")
+        #expect(first.sortOrder == 0)
+        #expect(second.sortOrder == 1)
+    }
+
+    @Test("listFolders returns folders ordered by sortOrder")
+    func listFoldersOrdered() throws {
+        let db = DatabaseService.makeInMemory()
+        _ = try db.createFolder(name: "Work")
+        _ = try db.createFolder(name: "Personal")
+
+        let folders = try db.listFolders()
+        #expect(folders.map(\.name) == ["Work", "Personal"])
+    }
+
+    @Test("renameFolder updates the stored name")
+    func renameFolderUpdatesName() throws {
+        let db = DatabaseService.makeInMemory()
+        let folder = try db.createFolder(name: "Work")
+        try db.renameFolder(id: folder.id, name: "Projects")
+
+        let folders = try db.listFolders()
+        #expect(folders.first?.name == "Projects")
+    }
+
+    @Test("moveConversation files a conversation into a folder")
+    func moveConversationFiles() throws {
+        let db = DatabaseService.makeInMemory()
+        let folder = try db.createFolder(name: "Work")
+        let conversation = try db.saveConversation(name: "Chat", messages: [Message(role: .user, content: "hi")])
+
+        try db.moveConversation(id: conversation.id, toFolder: folder.id)
+
+        let loaded = try db.loadConversation(id: conversation.id)
+        #expect(loaded?.0.folderId == folder.id)
+    }
+
+    @Test("moveConversation with nil folder unfiles a conversation")
+    func moveConversationUnfiles() throws {
+        let db = DatabaseService.makeInMemory()
+        let folder = try db.createFolder(name: "Work")
+        let conversation = try db.saveConversation(name: "Chat", messages: [Message(role: .user, content: "hi")])
+        try db.moveConversation(id: conversation.id, toFolder: folder.id)
+
+        try db.moveConversation(id: conversation.id, toFolder: nil)
+
+        let loaded = try db.loadConversation(id: conversation.id)
+        #expect(loaded?.0.folderId == nil)
+    }
+
+    @Test("Deleting a folder unfiles its conversations instead of deleting them")
+    func deleteFolderUnfilesConversations() throws {
+        let db = DatabaseService.makeInMemory()
+        let folder = try db.createFolder(name: "Work")
+        let conversation = try db.saveConversation(name: "Chat", messages: [Message(role: .user, content: "hi")])
+        try db.moveConversation(id: conversation.id, toFolder: folder.id)
+
+        try db.deleteFolder(id: folder.id)
+
+        let folders = try db.listFolders()
+        #expect(folders.isEmpty)
+
+        let loaded = try db.loadConversation(id: conversation.id)
+        #expect(loaded != nil)
+        #expect(loaded?.0.folderId == nil)
+    }
+
+    @Test("listConversations reflects folderId")
+    func listConversationsReflectsFolderId() throws {
+        let db = DatabaseService.makeInMemory()
+        let folder = try db.createFolder(name: "Work")
+        let conversation = try db.saveConversation(name: "Chat", messages: [Message(role: .user, content: "hi")])
+        try db.moveConversation(id: conversation.id, toFolder: folder.id)
+
+        let conversations = try db.listConversations()
+        #expect(conversations.first?.folderId == folder.id)
+    }
+}