Add Google Analytics tracking to application pages

[Copilot]

Implements Google Analytics 4 (GA4) tracking across the application to enable user analytics.

Changes

• HTML files: Added gtag.js tracking script to src/index.html and src/login.html with placeholder tracking ID G-XXXXXXXXXX
• Configuration: Documented GA_TRACKING_ID in .env.example for reference
• Documentation: Added setup instructions in README for obtaining and configuring Measurement ID

Implementation

<!-- Google Analytics -->
<!-- Replace BOTH occurrences of 'G-XXXXXXXXXX' below with your actual tracking ID -->
<script async src="https://www.googletagmanager.com/gtag/js?id=G-XXXXXXXXXX"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());
  gtag('config', 'G-XXXXXXXXXX');
</script>

Users replace the placeholder tracking ID in both HTML files, then rebuild with npm run build to activate tracking.

Original prompt

Work on TODO: Add Google Analytics --> (from src/index.html)

Created from VS Code via the GitHub Pull Request extension.

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