feat: add runtime tracking control with privacy features

[sadjow]

Summary

Introduces runtime tracking control for dynamic enable/disable based on user consent (e.g., cookie banners).

Closes #28

Features

• Runtime Control: Enable/disable tracking dynamically via useNuxtUTM() composable
• Privacy Focused: Clear all data, persistent preferences, GDPR-friendly
• Cookie Banner Ready: Perfect for consent management integration

Breaking Changes

useNuxtUTM() now returns an object instead of data array:

// Before
const utm = useNuxtUTM() // data array

// After  
const utm = useNuxtUTM()
const data = utm.data.value

API

const utm = useNuxtUTM()
// utm.data - collected UTM data
// utm.trackingEnabled - current state
// utm.enableTracking() 
// utm.disableTracking()
// utm.clearData()

Test Plan

• 41 tests passing
• Runtime control test suite
• Interactive playground demo