feat(analytics): GA4 tracking for presets.polymaker.com downloads

[DerTeam-Polymaker]

Summary

• Add GA4 base tag (gtag.js) to index.html with Measurement ID G-KXL1MCXGJZ
• Instrument download flows in app.js with four custom events fired on successful download
• Enable debug_mode on localhost / 127.0.0.1 only to support local DebugView validation
• Scope is limited to the presets.polymaker.com download page (index.html + app.js)

Events

Event	Trigger
download_single	Single JSON download succeeds (non-Bambu direct + Bambu JSON zip)
download_bundle	Single .bbsflmt download succeeds
download_selected	Multi-select ZIP download succeeds
download_bundle_batch	Multi-material bundle ZIP download succeeds

Event parameters

• Common: file_type, slicer, material, brand, model
• Batch: selected_count, series, material_count
• Missing values fallback to unknown; no PII is sent

Local validation (completed)

Verified in GA4 DebugView on localhost:

• page_view
• download_single
• download_bundle
• download_selected
• download_bundle_batch

Post-deploy checklist

• Deploy to https://presets.polymaker.com
• Verify events in GA4 Realtime within 30 minutes
• Mark the four download events as Key events in GA4 Admin
• Confirm yellow "Data collection isn't active" warning clears after traffic arrives (24–48h)

Test plan

• Open production site and confirm page_view in Realtime
• Trigger one download per event type and confirm event names + parameters
• Confirm download behavior is unchanged (files still download correctly)
• Confirm non-Bambu JSON download still works after switching to fetch-based download for tracking