feat(app): add jump-to-top and jump-to-bottom chat shortcuts

[anduimagui]

Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This is a small UX upgrade for chat navigation. It makes moving around long threads feel more like normal web scrolling habits, without changing the core message flow.

We already had commands for previous/next user message. This adds quick jump-to-top and jump-to-bottom commands so it’s easier to move through long AI chats and get back to where you need to be fast.

Added:

• mod+shift+arrowup = jump to top
• mod+shift+arrowdown = jump to bottom
• desktop menu items + docs update

How did you verify your code works?

• Checked command wiring and keybind behavior
• Checked desktop menu items and accelerators
• Checked docs update for new shortcuts

Screenshots / recordings

UI change — will attach a short recording of:

1. jump to top
2. jump to bottom
3. previous/next user-message navigation still working

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR