fix: replace semaphore deadlock pattern in SSH host key verification

[datlechin]

Summary

• HostKeyVerifier — Replaced DispatchSemaphore.wait() + DispatchQueue.main.async { alert.runModal() } pattern with proper async/await + @MainActor + beginSheetModal(for:). Eliminates thread blocking on Swift concurrency cooperative pool.
• LibSSH2TunnelFactory — Made createTunnel, testConnection, buildAuthenticatedChain, and verifyHostKey all async throws to support the async verification flow.
• SSHTunnelManager — Removed Task.detached wrappers (no longer needed since factory methods are async).
• Fixed 2 String(localized:) with interpolation violations in host key prompt messages.

Addresses audit item C1 — the last remaining critical thread-safety issue.

Test plan

• Connect to SSH server NOT in known_hosts → host key prompt appears as a sheet on the key window
• Accept unknown host key → connection succeeds, key saved to known_hosts
• Cancel unknown host key prompt → connection fails gracefully
• Connect to SSH server with changed key → mismatch warning sheet appears
• Accept changed key → connection succeeds, key updated
• Cancel changed key prompt → connection fails
• Connect to known SSH server → no prompt, connects directly
• Connect through jump hosts → each hop prompts if unknown
• Test SSH connection from Connection Form → works correctly
• No UI freeze during host key prompts