Feature: Add UI for ghost text previews and interactive code diffs

[AdamFrenzen]

Feature:

Add the UI for AI-generated code diffs and ghost text previews. Users can preview suggestions as ghost text, and accept or reject code changes in an interactive diff UI.

Design:

Ghost text + code diffs, inspired by modern code suggestion tools.

Notes:

• Will likely use nui.line for rendering diff lines and ghost text overlays
• Should integrate with existing inline prompt system
• May extend to automatically suggest fixes when inline errors are detected

[AdamFrenzen]

Update:

I’ve implemented the inline diff rendering using virtual lines and extmarks instead of the original plan with nui.line. This approach gives more control over highlighting and keeps the UI tightly integrated with the actual code lines.

Up next:

Adding keybinds to accept/reject the suggested changes, which will send the decision back to the Rust backend for applying or discarding the edits.