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/decisions/ADR-025-indieauth-pkce-authentication.md

@@ -202,7 +202,7 @@ The technical implementation is documented in:
 ### Supporting Specifications
 - **PKCE Specification (RFC 7636)**: https://www.rfc-editor.org/rfc/rfc7636
 - **OAuth 2.0 (RFC 6749)**: https://www.rfc-editor.org/rfc/rfc6749
-- **IndieAuth Specification**: https://indieauth.spec.indieweb.org/ (context only)
+- **IndieAuth Specification**: https://www.w3.org/TR/indieauth/ (context only)
 
 ### Internal Documentation
 - ADR-005: IndieLogin Authentication Integration (conceptual flow)