ui: export full message tree instead of active path only (#25501)

ui: export full message tree instead of active path only (#25501)

downloadConversation serialized activeMessages, the root -> currNode
path, so exporting a conversation with edited or regenerated messages
dropped every alternate version and kept only the selected one.

Fetch the whole message tree via getConversationMessages so the export
carries all message versions, matching the multi-conversation export
path which already did this. Keep the active conversation as the header
source to preserve an up-to-date currNode.

Forks are separate conversations, each with its own convId, and are
exported on their own.