Update app chrome responsiveness and make heatmap intensity relative to current overlap

[felixgollnhuber]

Summary

• lock browser metadata branding to tempoll.app for default and templated page titles
• compact the app chrome controls for small screens with icon-first actions while preserving accessible labels
• add a mobileIcon mode to LanguageSwitcher with locale label fallback on larger screens
• switch heatmap color intensity from absolute counts to relative scaling against the current max overlap
• align heatmap legend swatches with the same relative overlap scale
• add/adjust tests for compact header accessibility, language switcher icon rendering, and relative heatmap coloring

Testing

• pnpm lint
• pnpm typecheck
• pnpm test:run src/components/public-event-client.test.tsx