refactor: deduplicate URL generation and standardize on localhost

- Extract duplicated URL generation code into getClientUrl() helper function in start.js
- Replace all 127.0.0.1 references with localhost for consistency across codebase
- Update server to respect HOST environment variable for URL generation
- Remove 127.0.0.1 from default allowed origins in CORS configuration
- Update documentation to use localhost instead of 127.0.0.1

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

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

client/bin/client.js

@@ -40,7 +40,7 @@ const server = http.createServer((request, response) => {
 });
 
 const port = process.env.PORT || 6274;
-const host = process.env.HOST || "127.0.0.1";
+const host = process.env.HOST || "localhost";
 server.on("listening", () => {
   console.log(
     `🔍 MCP Inspector is up and running at http://${host}:${port} 🚀`,