Allow custom SSH tunnel endpoint hosts

[ZacharyZcR]

Summary

• let server tunnel endpoint hosts be typed directly instead of limited to SSH-host selections
• keep existing SSH host suggestions so managed S2S tunnels still work when a saved SSH host is selected
• trim custom endpoint values before connecting so direct source-host forwarding reaches arbitrary hosts/IPs behind the bastion

Verification

• npx tsc --noEmit
• npm run lint
• npm run build
• npx prettier --check .
• git diff --check

Fixes Termix-SSH/Support#889