feat: stamp component source with docs-page signature

[starc007]

What

Adds a one-line provenance comment to component source — both when copied from the docs and when installed via the registry — so the origin travels with the code:

"use client";
// beui.dev/components/motion/tabs
import ...

Plus centralizes the site domain behind an env var.

Signature

• lib/signature.ts
  • withSignature(content, path, pageUrl) — inserts the banner after any use client/use server directive, else at the top. Picks // vs /* */ by extension. No-ops non-code files (json, etc.).
  • pageUrlFor(category, slug) — the docs page URL.
• Applied to the component's own source files only (not shared util/lib deps):
  • Install — buildEntry + buildShadcnItem in lib/registry-server.ts
  • Copy — source tab on the component page (ExampleBlock + DefaultTabs). Usage/preview tabs unchanged, matching what installs ship.

Domain from env

• lib/site.ts — SITE_URL = NEXT_PUBLIC_SITE_URL ?? "https://beui.dev" (trailing slash stripped).
• Reused in registry-server, robots, sitemap, and layout metadataBase (previously hardcoded https://beui.dev in each).
• NEXT_PUBLIC_SITE_URL is optional; the fallback keeps prod working. Set it for non-prod envs to self-reference.

Verification

• bun run typecheck clean
• bun run check:registry validates 35 components
• bun run lint clean (one pre-existing warning in install-command.tsx, unrelated)