AboutModal unreachable on Linux/Windows

[stultus]

Problem

AboutModal is discoverable only via the native macOS application menu, which:

• Linux and Windows users never see (their menu layout differs).
• New users don't know to look in the app menu for an About screen.

Observed

• No in-app link (status bar, Help modal, Settings) routes to About.
• On non-mac platforms this means the About screen is effectively hidden.

Proposed

• Add an "About Scriptty" link in HelpModal (footer of the shortcuts list, or its own
  section).
• Optionally add a small version number in the Settings modal footer that opens
  AboutModal on click.
• Keep the menu item for macOS users who expect it there.

Acceptance

• About is reachable from inside the app on all platforms.
• Version number is visible to users who want to report bugs.