[UI] Old UI on new event loop

[dingfeli]

In preparation for the new event loop, which is to be written decoupled from the visual layer, we need to preserve the same UX while the new UI is underway.

There are the following todos to be completed in order for this to be achieved:

• UI managed input (chore: legacy UI for new event loop #3254)
• Slash command (autocompletion + structured event egress)
• Skim integration
• MCP prompts (autocompletion + structured event egress)
• History integration
• Image pasting integration (Pasting of path + pasting of image from clipboard)

Make sure to comment on this issue (with a draft PR if applicable) if you are working on any of these.