feat(terminal): paste selected text

[St0ff3l]

Summary

• Add a terminal context menu action that pastes the current xterm selection into the active terminal input.
• Keep selected text in the context-menu snapshot; do not access the OS clipboard or append Enter.
• Add Chinese and English labels and document the behavior.

Validation

• Typecheck, lint, formatting, renderer and Rust tests, Clippy, and CSS contract check passed locally.