Integrate credentials manager with host authentication system

- Add credential selection tab to Host Manager editor
- Create CredentialSelector component for credential selection
- Update form validation to support credential-based authentication
- Extend SSH host interfaces with credentialId field
- Add internationalization support for credential authentication
- Fix OIDC config endpoint to return 200 with null data instead of 404
- Improve credentials UI spacing and color consistency
- Remove hardcoded colors and use Zinc theme throughout

electron/main-simple.cjs

@@ -21,7 +21,8 @@ function startBackendServer() {
 
     backendProcess = spawn('node', [backendPath], {
         stdio: ['ignore', 'pipe', 'pipe'],
-        detached: false
+        detached: false,
+        cwd: path.join(__dirname, '..')  // Set working directory to app root
     });
 
     backendProcess.stdout.on('data', (data) => {