Summary

• Add collapsible file headers to the diff panel so each file can be expanded or collapsed independently.
• Show file change type badges and accepted state directly in the header, with quick actions to accept or open a file in the editor.
• Update the code viewer action bar so discard is only shown for dirty tabs and refine save/discard button styling.

Testing

• Not run (per request).
• Visual check the diff panel header interactions for collapse/expand, accept, and editor open behavior.
• Verify dirty code tabs still show Discard while clean tabs do not.