feat: add live countdown timer and early bird ticket alert modal

[esrakadah]

Summary of Changes

1. Interactive Live Countdown Timer:

   • Added live countdown timer targeting DevFest Berlin 2026 (2026-11-14T09:00:00+01:00) in the hero section.
   • Designed with frosted glassmorphism cards, glowing Google Blue numbers, tabular figures, and pulsing separator colons.
   • Fully responsive down to 320px screens.

2. Early Bird Notification CTA & Modal Dialog:

   • Added prominent Early Bird Alert CTA in the hero action cluster (shown when ticket sales are not yet open).
   • Integrated native HTML5 <dialog> modal with focus trap, backdrop blur, Escape key dismissal, backdrop-click dismissal, and accessible close button.
   • Connected directly to newsletter endpoint with instant inline feedback and auto-dismissal on success.
   • Full dark mode styling.

3. Negative Constraint Check:

   • Verified 0 mentions of Hackathon or Vibe Lounge.