Add terminal copy and paste shortcuts

[bifrost0x]

Summary

• add platform-aware copy and paste handling to each xterm instance
• preserve Ctrl+C as the SSH interrupt when no text is selected
• keep paste on xterm's native path, including newline normalization and bracketed paste
• document the shortcuts and refresh the terminal-manager asset version

Why

This implements the request from Discussion #40 without changing the backend, Socket.IO input path, or dependencies.

Validation

• node --check static/js/terminal-manager.js
• node --check static/js/app.js
• pytest tests -q: 137 passed, 27 skipped
• browser verification for selection copy, Ctrl+C interrupt, multiline paste, and bracketed paste
• local Docker image build and healthy container startup