fix(landing): disable analytics outside production

[owjs3901]

Summary

• Initialize Google Tag Manager only on devup-ui.com.
• Remove the no-JavaScript GTM fallback iframe.
• Add localhost regression coverage for blocked analytics requests.

Validation

• bun tsc --noEmit -p apps/landing/tsconfig.json
• bun eslint apps/landing/src/app/layout.tsx e2e/helpers.ts e2e/landing-build-integrity.spec.ts e2e/landing-zero-runtime.spec.ts
• bunx playwright test e2e/landing-build-integrity.spec.ts --grep "Google Tag Manager is disabled"
• bunx playwright test e2e/docs-pages.spec.ts --grep "header has Docs link"

Note: the repository pre-commit coverage stage could not complete in this Windows environment because Cargo reported a closed-pipe/cache dependency error for zerocopy_derive.

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