[TUI] Add Ctrl+Y shortcut to copy composer draft to clipboard

What feature would you like to see?

Add a dedicated shortcut to copy the current TUI composer draft to the system clipboard.

Proposed behavior

• Press Ctrl+Y in the TUI composer to copy the current draft text.
• Show a short footer confirmation (for example: Copied composer draft to clipboard.).
• If clipboard access fails, surface a clear error message in history/status.

Why this helps

When iterating on prompts, users often want to:

• copy a partially-written draft to an editor,
• snapshot a draft before rewriting,
• move draft text between sessions.

Today this is cumbersome and terminal-dependent (manual selection/mouse copy/etc.). A dedicated shortcut would make this fast and consistent.

Scope notes

• This is specifically for composer draft text (not transcript export).
• Keeping the shortcut discoverable in the footer shortcut list would help adoption.

Additional information

Related, but not the same request:

• #9132 (terminal-like editing expectations)
• #2880 (copy/export message as markdown)