feat(webui): render Write and Edit tool diffs

[Premsenareddy]

Summary

• add a shared diff preview renderer for Write/Edit tool results
• render unified git-style diffs with react-diff-view and unidiff
• collapse large diffs behind an expandable "N more lines" banner
• preserve default output for non-success states so errors remain visible

Closes #1846

Tests

• corepack pnpm exec prettier --check frontend/src/components/chat/tool-renderers/DiffPreview.tsx frontend/src/components/chat/tool-renderers/EditRenderer.tsx frontend/src/components/chat/tool-renderers/WriteRenderer.tsx frontend/src/types/unidiff.d.ts frontend/package.json pnpm-lock.yaml
• corepack pnpm --filter frontend build
• corepack pnpm --filter frontend lint
  • passes with existing warnings in unrelated files