Implement Google Consent Mode V2

[evulins]

Summary

• Upgrade cookie consent to Google Consent Mode V2, sending all 4 required params (analytics_storage, ad_storage, ad_user_data, ad_personalization) in a single gtag('consent', 'update', ...) call
• Load GTM unconditionally with all consent signals defaulted to denied, then restore saved preferences from localStorage before GTM fires
• Guard against undefined sessions in fixedSessionsSelectors to prevent runtime crash
• Update cookieManager tests to cover all V2 consent parameter combinations

Trello task

Test plan

• Verify GTM loads on page load with consent defaults set to denied
• Accept analytics only - confirm analytics_storage granted, ad params denied
• Accept marketing only - confirm ad params granted, analytics_storage denied
• Accept all - confirm all 4 params granted
• Deny all - confirm all 4 params denied and cookies are cleared
• Verify consent state persists across page reloads via localStorage

[hanka]

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