ui: IndexedDB and Conversations data fixes (#26278)

ui: IndexedDB and Conversations data fixes (#26278)

* fix: single-flight conversations store init

* refactor: remove unused legacy-migration util

* fix: make createSystemMessage transactional

* fix: delete message branches cascading on edit/regenerate

* fix: stop stamping lastModified on conversation metadata updates

* fix: count cascaded forks in bulk delete toast, bulkify deleteAll

* refactor: drop redundant conversation list respreads

* refactor: create conversation in a single write

* fix: use table constant in toggleConversationPin

* fix: keep the system message placeholder out of the edit form

* fix: keep focus in the system message editor after opening it

* fix: focus the main chat form after submitting a system message

* fix: update timestamp of the correct conversation on stream completion