Summary

• Fixes TUI /copy on WSL/Windows when text is written through PowerShell.
• Sets PowerShell stdin decoding to UTF-8 before reading piped clipboard text.
• Keeps the existing Linux/macOS clipboard fallback order unchanged.

Why

WSL sends /copy text to powershell.exe Set-Clipboard over stdin. On systems with non-UTF-8 ANSI code pages, PowerShell can decode those UTF-8 bytes as the local code page before writing the Windows clipboard, which garbles CJK and other non-ASCII text.

Changes

• Replace the $input PowerShell write path with [Console]::In.ReadToEnd() after setting [Console]::InputEncoding to UTF-8.
• Reuse the same UTF-8 PowerShell command for native Windows and WSL clipboard writes.
• Add clipboard unit assertions covering the UTF-8 PowerShell write command.

Validation

• cd ui-tui && npx eslint src/lib/clipboard.ts src/__tests__/clipboard.test.ts
• cd ui-tui && npm test -- --run src/__tests__/clipboard.test.ts

Scope

Focused on TUI clipboard writes through PowerShell. Clipboard reads and non-PowerShell Linux/macOS backends are unchanged.

Closes #35107