fix(desktop): improve conversation performance and editing UX

[Chal1ce]

Summary

Improve the Desktop conversation experience in three areas:

• Reduce rendering work during long streaming conversations.
• Allow editing and replacing only the latest user message in place.
• Keep live reasoning and tool activity compact and user-expandable.

Changes

• Coalesce high-frequency streaming events per animation frame.
• Memoize conversation message rows and avoid unnecessary preview IPC calls.
• Add Chromium content-visibility for historical messages.
• Add /edit-last to replace the latest user message in the same session.
• Do not create a fork or a new session when editing.
• Remove the old assistant/tool response when the edited message is resent.
• Add inline editing, copy, cancel, and resend actions for user messages.
• Show message actions below the user message on hover.
• Display only the latest reasoning/tool activity line while the model is working.
• Keep reasoning and tool details collapsed by default and expandable on demand.
• Preserve automatic scroll-to-tail after editing and resending a message.