fix: use configured site URL for copy, share, and approval links

[slegarraga]

Replaces hardcoded https://agentpostmortem.com URLs with getSiteUrl() in user-facing and admin flows.

• Copy Link now respects NEXT_PUBLIC_SITE_URL / NEXT_PUBLIC_APP_URL during local development.
• The X share link uses the same configured origin.
• Approval emails use the configured origin for the case URL.

Verified with npm run lint, npx tsc --noEmit, Prettier, and the test suite.

Closes #2

[royalpinto007]

Great second contribution, thank you again!

This is the right call in all three spots. I especially like that you caught the admin approval email in app/api/admin/posts/[id]/route.ts and the X share link, not just the copy button the issue mentioned. Those would have kept sending people to the hardcoded origin.

Worth noting for anyone reading later: getSiteUrl() only reads NEXT_PUBLIC_* vars, which Next inlines at build time, so it works correctly inside the client CopyLinkButton and still falls back to production when nothing is configured.

All checks green. Merging.