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/Services/DatabaseService.swift

@@ -35,6 +35,16 @@ struct ConversationRecord: Codable, FetchableRecord, PersistableRecord, Sendable
     var createdAt: String
     var updatedAt: String
     var primaryModel: String?
+    var folderId: String?
+}
+
+struct FolderRecord: Codable, FetchableRecord, PersistableRecord, Sendable {
+    static let databaseTableName = "folders"
+
+    var id: String
+    var name: String
+    var sortOrder: Int
+    var createdAt: String
 }
 
 struct MessageRecord: Codable, FetchableRecord, PersistableRecord, Sendable {
@@ -342,6 +352,20 @@ final class DatabaseService: Sendable {
             )
         }
 
+        migrator.registerMigration("v9") { db in
+            // Folders for organizing conversations
+            try db.create(table: "folders") { t in
+                t.primaryKey("id", .text)
+                t.column("name", .text).notNull()
+                t.column("sortOrder", .integer).notNull().defaults(to: 0)
+                t.column("createdAt", .text).notNull()
+            }
+
+            try db.alter(table: "conversations") { t in
+                t.add(column: "folderId", .text).references("folders", onDelete: .setNull)
+            }
+        }
+
         return migrator
     }
 
@@ -404,7 +428,7 @@ final class DatabaseService: Sendable {
         return try saveConversation(id: UUID(), name: name, messages: messages, primaryModel: nil)
     }
 
-    nonisolated func saveConversation(id: UUID, name: String, messages: [Message], primaryModel: String?) throws -> Conversation {
+    nonisolated func saveConversation(id: UUID, name: String, messages: [Message], primaryModel: String?, folderId: UUID? = nil) throws -> Conversation {
         Log.db.info("Saving conversation '\(name)' with \(messages.count) messages (primaryModel: \(primaryModel ?? "none"))")
         let now = Date()
         let nowString = Self.isoString(from: now)
@@ -414,7 +438,8 @@ final class DatabaseService: Sendable {
             name: name,
             createdAt: nowString,
             updatedAt: nowString,
-            primaryModel: primaryModel
+            primaryModel: primaryModel,
+            folderId: folderId?.uuidString
         )
 
         let messageRecords = messages.enumerated().compactMap { index, msg -> MessageRecord? in
@@ -446,7 +471,8 @@ final class DatabaseService: Sendable {
             messages: savedMessages,
             createdAt: now,
             updatedAt: now,
-            primaryModel: primaryModel
+            primaryModel: primaryModel,
+            folderId: folderId
         )
     }
 
@@ -526,7 +552,8 @@ final class DatabaseService: Sendable {
                 messages: messages,
                 createdAt: createdAt,
                 updatedAt: updatedAt,
-                primaryModel: convRecord.primaryModel
+                primaryModel: convRecord.primaryModel,
+                folderId: convRecord.folderId.flatMap { UUID(uuidString: $0) }
             )
 
             return (conversation, messages)
@@ -568,7 +595,8 @@ final class DatabaseService: Sendable {
                     messages: Array(repeating: Message(role: .user, content: ""), count: messageCount),
                     createdAt: createdAt,
                     updatedAt: lastDate,
-                    primaryModel: primaryModel
+                    primaryModel: primaryModel,
+                    folderId: record.folderId.flatMap { UUID(uuidString: $0) }
                 )
                 conv.updatedAt = lastDate
                 return conv
@@ -576,6 +604,68 @@ final class DatabaseService: Sendable {
         }
     }
 
+    // MARK: - Folders
+
+    nonisolated func createFolder(name: String) throws -> Folder {
+        let folder = Folder(name: name, sortOrder: try nextFolderSortOrder())
+        let record = FolderRecord(
+            id: folder.id.uuidString,
+            name: folder.name,
+            sortOrder: folder.sortOrder,
+            createdAt: Self.isoString(from: folder.createdAt)
+        )
+        try dbQueue.write { db in
+            try record.insert(db)
+        }
+        return folder
+    }
+
+    private nonisolated func nextFolderSortOrder() throws -> Int {
+        try dbQueue.read { db in
+            let row = try Row.fetchOne(db, sql: "SELECT MAX(sortOrder) AS maxOrder FROM folders")
+            let maxOrder: Int? = row?["maxOrder"]
+            return (maxOrder ?? -1) + 1
+        }
+    }
+
+    nonisolated func renameFolder(id: UUID, name: String) throws {
+        try dbQueue.write { db in
+            try db.execute(
+                sql: "UPDATE folders SET name = ? WHERE id = ?",
+                arguments: [name, id.uuidString]
+            )
+        }
+    }
+
+    nonisolated func deleteFolder(id: UUID) throws {
+        try dbQueue.write { db in
+            try FolderRecord.deleteOne(db, key: id.uuidString)
+        }
+    }
+
+    nonisolated func listFolders() throws -> [Folder] {
+        try dbQueue.read { db in
+            let records = try FolderRecord
+                .order(Column("sortOrder"))
+                .fetchAll(db)
+            return records.compactMap { record -> Folder? in
+                guard let id = UUID(uuidString: record.id),
+                      let createdAt = Self.isoDate(from: record.createdAt)
+                else { return nil }
+                return Folder(id: id, name: record.name, sortOrder: record.sortOrder, createdAt: createdAt)
+            }
+        }
+    }
+
+    nonisolated func moveConversation(id: UUID, toFolder folderId: UUID?) throws {
+        try dbQueue.write { db in
+            try db.execute(
+                sql: "UPDATE conversations SET folderId = ? WHERE id = ?",
+                arguments: [folderId?.uuidString, id.uuidString]
+            )
+        }
+    }
+
     // MARK: - Usage Statistics
 
     nonisolated func getOverallUsageStats() throws -> UsageStats {