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
