Bound long-line previews in edit and write tool cards #59

Part of #2

Edit and write input previews currently show an entire first diff row when it exceeds the 2,000-character preview limit. A single minified or generated line can therefore bypass truncation.

Bound the rendered diff preview even within a line, explain when the preview is shortened, and retain full-input copying and Show more / Show less. Add component regressions and real Electron coverage with screenshot proof. Keep the parent open.