ui: Services consolidation refactor (#27239)
* ui: Move stream lookup and replay fetches into ChatService
chatStore called fetch() directly for /v1/streams/lookup and the
/v1/stream replay. These now live next to the other stream-session
methods in ChatService, so services stay the only API I/O layer.
* ui: Move /models/sse feed reader into ModelsService
ModelsService.watchModelEvents owns the byte stream, reconnect loop
and SSE record parsing; modelsStore keeps only event routing and
state.
* ui: Extract conversation import/export into ConversationTransferService
The JSONL session format, ZIP archiving and browser downloads are
pure I/O with no store state, so they move out of
conversationsStore. The store keeps the DB orchestration
(bulkExportConversations, downloadConversation,
importConversationsData) and delegates the format work.
* ui: Consolidate active model resolution into modelsStore.activeModelId
The same resolution chain was duplicated in useChatScreenActiveModel,
ChatForm, ChatFormActionModels and contextStatsStore, with slight
drift in the single-model fallback. The canonical getter now lives in
modelsStore, and the shared last-assistant-model lookup moved to
utils as getConversationModel.
* ui: Initialize stores explicitly via initStores()
Store constructors and module-level side effects ran migrations and
localStorage reads in import order. Migrations rename and rewrite
localStorage keys, so a settings load racing ahead of them could
clobber migrated values. initStores() is called once from the root
layout and runs migrations first, then the stores that read
localStorage, then the conversations DB load.
* refactor: Constants for stream query params
