feat: redesign site with Astro and consolidate industries

Migrate from Phoenix/Elixir to Astro static site with Preact islands.

Industry consolidation:
- Consolidate 89 freeform industry values into 17 validated categories
- Add multi-industry support (1-3 industries per company via Zod enum array)
- Add 4 new categories: Professional Services, Media & Entertainment,
  Government & Non-Profit, Agriculture & Food
- All 326 company markdown files migrated with primary/secondary industries
- Build-time validation ensures no invalid industry values

New Astro site includes:
- Static site generation with content collections
- Preact search/filter island with URL state sync
- Tailwind CSS with dark mode support
- Company cards with industry color coding and icons
- Related companies based on industry overlap

Author: doomspork