webui: Conversation forking + branching improvements (#21021)
* refactor: Make `DialogConfirmation` extensible with children slot
* feat: Add conversation forking logic
* feat: Conversation forking UI
* feat: Update delete/edit dialogs and logic for forks
* refactor: Improve Chat Sidebar UX and add MCP Servers entry
* refactor: Cleanup
* feat: Update message in place when editing leaf nodes
* chore: Cleanup
* chore: Cleanup
* chore: Cleanup
* chore: Cleanup
* chore: Cleanup
* chore: Cleanup
* refactor: Post-review improvements
* chore: update webui build output
* test: Update Storybook test
* chore: update webui build output
* chore: update webui build output
