FIX: Resolve critical reverse proxy security vulnerability and complete i18n implementation

Security Fixes:
- Configure Express trust proxy to properly detect client IPs behind nginx reverse proxy
- Remove deprecated isLocalhost() function that was vulnerable to IP spoofing
- Ensure /ssh/db/host/internal endpoint uses secure token-based authentication only

Internationalization Improvements:
- Replace hardcoded English strings with proper i18n keys in admin settings
- Complete SSH configuration documentation translation (sshpass, server config)
- Add missing translation keys for Debian/Ubuntu, macOS, Windows installation methods
- Fix Chinese translation key mismatches for SSH server configuration options

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/locales/en/translation.json

@@ -570,6 +570,8 @@
     "sshpassRequired": "Sshpass Required For Password Authentication",
     "sshpassRequiredDesc": "For password authentication in tunnels, sshpass must be installed on the system.",
     "otherInstallMethods": "Other installation methods:",
+    "debianUbuntuEquivalent": "(Debian/Ubuntu) or the equivalent for your OS.",
+    "or": "or",
     "centosRhelFedora": "CentOS/RHEL/Fedora",
     "macos": "macOS",
     "windows": "Windows",