docs: Standardize all IndieAuth spec references to W3C URL

- Updated 42 references from indieauth.spec.indieweb.org to www.w3.org/TR/indieauth
- Ensures consistency across all documentation
- Points to the authoritative W3C specification
- No functional changes, documentation update only

Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

docs/design/phase-3-authentication-implementation.md

@@ -534,7 +534,7 @@ After Phase 3 completion:
 
 - [ADR-005: IndieLogin Authentication](/home/phil/Projects/starpunk/docs/decisions/ADR-005-indielogin-authentication.md)
 - [ADR-010: Authentication Module Design](/home/phil/Projects/starpunk/docs/decisions/ADR-010-authentication-module-design.md)
-- [IndieAuth Specification](https://indieauth.spec.indieweb.org/)
+- [IndieAuth Specification](https://www.w3.org/TR/indieauth/)
 - [IndieLogin API Documentation](https://indielogin.com/api)
 - [OWASP Authentication Cheat Sheet](https://cheatsheetseries.owasp.org/cheatsheets/Authentication_Cheat_Sheet.html)