v6.1 - Dashboard enhancements, caching improvements, and bug fixes

Features

• Replace submission funnel with time-to-submit trend chart
• Add entity confirmation rates and statistics to dashboard
• Add new stats components for Overall and Extraction Stats pages
• Remove 'Additional type of expression data' field from form
• Add troubleshooting instructions and hard refresh guidance
• Implement internal link handling in FAQ component
• Enhance FAQ answers with italicized text

Fixes

• Remove TinyURL dependency to avoid interstitial redirect page
• Bundle and self-host Bootstrap CSS (avoids CDN blocking in China)
• Move CSS into webpack pipeline for content-hashed URLs
• Add API cache headers and improve nginx caching strategy
• Fix response rate calculation for zero/null values
• Defensive handling for null person with better error messages