Inspect the end of long tool output without expanding it all #66

Part of #2.

Long tool output currently previews only the beginning, which hides final errors and completion messages unless the entire result is expanded.

Add a bounded end preview to tool output cards, with an explicit control to return to the beginning. Keep full expansion and exact copying available. While a tool streams, the end preview should follow appended output. Input inspection stays independent.

Verify failed and streaming output, switching between preview modes, full expansion and copying through component tests. Add an Electron interaction and screenshot of the changed card.