[Feature] Display keyboard shortcut hint (like exit shortcuts) in login selection menu and other Tui menus

[Chi-Shan0707]

Description:
When users first run agy, they encounter the login selection menu:

Welcome to the Antigravity CLI. You are currently not signed in.

Select login method:
 > 1. Google OAuth
   2. Use a Google Cloud project

 [Use arrow keys to navigate, Enter to select]

Users expect Ctrl+C or ESC to exit/cancel, but neither works.
However, Ctrl+D does exit the menu, but there's no indication of this.
This causes users to feel "stuck" and unable to escape the menu.

Suggested Solution:
Add a subtle hint at the bottom of the menu:

Press Ctrl+D to exit

Or add it to the help/hints section, such as:

 [Use arrow keys to navigate, Enter to select, Ctrl+D to exit]

Why this matters:

• Most CLI tools (gh, gcloud, aws-cli) either support Ctrl+C or display exit instructions
• New users shouldn't have to guess or feel trapped
• Better discoverability of keyboard shortcuts improves overall UX
• This small addition can significantly reduce user frustration

Related:
Discovered while testing authentication flow on WSL2

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