Add native SSH theme clones and selectable key directories

[yaleman]

Summary

• Add native SSH transport for theme installs and updates without relying on an external ssh binary
• Introduce WEBSITES_THEME_SSH_KEY_DIR and store only the selected key filename for each theme
• Add theme edit routes and UI so admins can update repo, branch, and SSH key selection
• Record TOFU host keys in runtime data and document the new SSH flow

Testing

• Added and updated unit, web, and SSH integration coverage for key enumeration, key-name validation, edit/save behavior, and SSH cloning
• Verified the change set with the repo’s standard Rust, frontend, and Python client checks

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	cargo/​russh@​0.54.5

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[yaleman]

rust test failing due to mise/aqua pnpm being broken aquaproj/aqua-registry#52821 - works locally