feat(app): add git diff references to chat

[ipanasenko]

Adds new buttons in diff view to add files and ranges references to the last active chat.
Inspired by the same feature in Warp

• Clicking on a paperclip near file name adds file name
• Clicking inside diff adds file name with a range
• When chat is not in focus - swithes to the latest active tab with chat, or creates a new chat tab
• Works in both unified and split view
• Works on mobile (checked using Chrome device emulator)

Summary

• add file and hunk references from the Changes diff into the active chat composer or a new draft chat
• support unified and split diff reference targeting with shared reference-building utilities and tests
• refactor diff rendering and composer insertion helpers, including follow-up hover/scroll performance fixes for the paperclip gutter

This PR is not small, sorry about that.
I tried to make it smaller, but it was better to move some things around to reduce code duplication and improve organisation.

Hope this will be useful 🤞