refactor: move hardcoded URLs to environment variables

[silvioprog]

Summary

Refactors hardcoded URLs in index.html to use environment variables for better maintainability and consistency.

Changes

• Added cloudflareInsightsUrls configuration to env.config.ts
• Updated index.html to use __AUDIO_URL__ and __CLOUDFLARE_INSIGHTS_URL__ placeholders
• Enhanced vite-plugins/html.ts to replace URL placeholders with environment-specific values
• HTML plugin now captures build mode to select correct environment configuration

Benefits

• Centralizes URL configuration in env.config.ts
• Makes it easier to change URLs per environment
• Aligns audio URL usage with existing VITE_AUDIO_URL pattern
• Improves maintainability by removing hardcoded values from HTML

Files Changed

• env.config.ts - Added Cloudflare Insights URL configuration
• index.html - Replaced hardcoded URLs with placeholders
• vite-plugins/html.ts - Added URL placeholder replacement logic