fix: support PuTTY PPK ssh keys

[ZacharyZcR]

Summary

• allow uploaded PuTTY .ppk private keys through host and credential editors
• replace PEM-marker checks with shared SSH key parsing before terminal, SFTP, Docker, tunnel, tmux, and host metrics connections
• add coverage for ssh2-supported PuTTY PPK v2 keys and clearer errors for unsupported PPK formats

Refs Termix-SSH/Support#172

Validation

• npm run type-check
• npx eslint src/backend/utils/ssh-key-utils.ts src/backend/utils/ssh-key-utils.test.ts src/backend/database/routes/host.ts src/backend/database/routes/credentials.ts src/backend/database/routes/credential-deploy-routes.ts src/backend/ssh/terminal.ts src/backend/ssh/file-manager.ts src/backend/ssh/docker.ts src/backend/ssh/tunnel.ts src/backend/ssh/tmux-monitor.ts src/backend/ssh/host-metrics.ts src/ui/sidebar/HostEditor.tsx src/ui/sidebar/CredentialEditorView.tsx src/ui/ssh/dialogs/SSHAuthDialog.tsx
• git diff --check

Note: npm test -- src/backend/utils/ssh-key-utils.test.ts could not run in this checkout because the local install does not include the vitest binary.