[olufunsoolutayo]

Closes #406

What changed

• lib/env.ts validates the public environment once, at import time:
  • NEXT_PUBLIC_API_URL is required and must be an absolute http(s) URL.
  • NEXT_PUBLIC_SITE_URL and NEXT_PUBLIC_BOUNDLESS_APP_URL are optional (they keep their fallbacks in lib/seo/site.ts), but must be absolute http(s) URLs when set.
  • Any problem throws one message that names every bad variable and says to copy .env.example to .env.local.
• next.config.ts imports it, so both next build and next dev fail before compiling anything.
• lib/api/client.ts and lib/seo/site.ts read the values from publicEnv, so lib/env.ts is the only file that reads process.env.NEXT_PUBLIC_*. Trailing-slash trimming is unchanged.

Verification

Case	Result
npm run build with NEXT_PUBLIC_API_URL unset	fails, exit 1, names NEXT_PUBLIC_API_URL
NEXT_PUBLIC_API_URL=not-a-url npm run build	fails the same way
NEXT_PUBLIC_SITE_URL=ftp://x, NEXT_PUBLIC_BOUNDLESS_APP_URL=nope	fails, names both
npm run build with the .env.example values	passes
npm run dev without the variable	exits at startup with the same message

Error: Invalid or missing public environment variable(s): NEXT_PUBLIC_API_URL. Copy .env.example to .env.local and set each URL to an absolute http(s) URL.

npm run lint, npx tsc --noEmit, npm run build and npm run seo:check pass. The API origin is inlined into the client bundle and no chunk reads process.env at runtime.

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