feat: add support for SSH git URLs when installing themes

[Vargrul]

What this does

Adds support for installing Omarchy themes from SSH (scp-style) Git repository URLs and clarifies the accepted URL formats in the prompt.

Why

Previously, the theme installer only worked correctly with HTTPS Git URLs. Providing an SSH URL such as [email protected]:org/omarchy-foo-theme.git resulted in an incorrect theme name and install path.

Supporting SSH URLs makes it easier for contributors and users who rely on SSH-based Git workflows.

Changes

• Adds support for scp-style SSH Git URLs (git@host:org/repo.git) when deriving the theme name.
• Normalizes SSH URLs before calling basename to ensure correct theme directory naming.
• Updates the gum input placeholder to explicitly document supported Git URL formats.
• Keeps existing theme naming rules intact (omarchy- prefix and -theme suffix removal).

Examples

• https://github.com/org/omarchy-foo-theme.git → ~/.config/omarchy/themes/foo
• [email protected]:org/omarchy-foo-theme.git → ~/.config/omarchy/themes/foo

Notes

• No behavior changes for existing HTTPS-based installs.
• This change only affects how the repository name is parsed and does not modify cloning behavior.

[neheb]

shorter to use <<< instead of echo.