feat: add diff row wrapping

[quantk]

What changed

• add w toggle for wrapping long diff rows to the visible viewport
• retain full-row selection and change backgrounds across wrapped lines
• persist row-wrap state per review and restore it after F6 or reopen
• support the Russian-layout physical-key fallback
• document the shortcut in README and agent-guide

Validation

• npm run version:check
• npm test (41 Node tests and 10 Bun UI tests)
• npm run check
• git diff --check
• headless OpenTUI coverage verifies real virtual-line wrapping, viewport width, persistence, and toggle-off behavior