fix(gtm): invoke onBeforeGtmStart callback when ID is in config

[harlan-zw]

🔗 Linked issue

Resolves #522

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

When GTM ID is configured in nuxt.config.scripts.registry, the module generates a plugin that pre-initializes the script at app startup—before any user components mount. This meant onBeforeGtmStart callbacks passed later in components were never invoked since clientInit had already run with no callback in its closure.

Now after useRegistryScript returns, we check if window.gtag already exists (indicating pre-initialization) and immediately invoke the callback.

// nuxt.config - ID in config
scripts: {
  registry: {
    googleTagManager: { id: "GTM-XXXXXX" }
  }
}

// component - callback now works
useScriptGoogleTagManager({
  onBeforeGtmStart: (gtag) => {
    gtag('consent', 'default', { ad_storage: 'denied' })
  }
})

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Project	Deployment	Review	Updated (UTC)
scripts-docs	Ready	Preview, Comment	Jan 18, 2026 2:04am
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commit: 46f2e17