fix: validate server after resolution, not before (ISS-43 review)

Move cfg.Server = targetServer before ValidateForUp() so validation
checks the resolved value instead of the stale YAML value. Prevents
subtle bugs if ValidateForUp is extended to check additional fields.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: chuongld20

cmd/devbox/main.go

@@ -169,12 +169,10 @@ func upCmd(wm workspace.Manager) *cobra.Command {
 			}
 			// Tier 2: config.Server is already set in targetServer.
 
+			cfg.Server = targetServer
 			if err := cfg.ValidateForUp(poolConfigured); err != nil {
-				if targetServer == "" {
-					return fmt.Errorf("devbox up: %w", err)
-				}
+				return fmt.Errorf("devbox up: %w", err)
 			}
-			cfg.Server = targetServer
 
 			// Merge resource limits: server defaults <- workspace overrides.
 			globalCfg, err := config.LoadGlobal()