chore(phase0): prep cleanup — env example, dedup @types, fix biome jsx, remove autoprefixer

Phase 0a preparation fixes before major upgrades:

1. Create .env.example with all 21+ environment variables grouped by scope
   (public NEXT_PUBLIC_* vs server-only), with explanatory comments
2. Deduplicate @types/node, @types/react, @types/react-dom — removed from
   dependencies, kept in devDependencies with latest version ranges
3. Fix biome.json JSX runtime: reactClassic → automatic (for React 19)
4. Remove autoprefixer from dependencies (not needed with @tailwindcss/postcss
   in Tailwind v4)
5. components.json: tailwind.config.ts reference already removed (verified clean)

Co-authored-by: pm <pm@miriad.systems>

Author: Miriad

.env.example

@@ -2,6 +2,7 @@
 # codingcat.dev — Environment Variables
 # =============================================================================
 # Copy this file to .env.local and fill in the values.
+# See: https://nextjs.org/docs/app/building-your-application/configuring/environment-variables
 # Variables prefixed with NEXT_PUBLIC_ are exposed to the browser.
 # All other variables are server-only.
 # =============================================================================