Fix Proxmox import auth fallback

[ZacharyZcR]

Summary

• avoid importing Proxmox guests as password/key auth when no secret is available
• keep credential auth when a default or discovered credential exists
• add regression coverage for Proxmox import auth resolution

Test

• npx vitest run src/ui/components/proxmox/proxmox-import-auth.test.ts
• npm run type-check

Fixes Termix-SSH/Support#898

[LukeGus]

LGTM, thanks