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/reports/indieauth-client-discovery-analysis.md

@@ -242,7 +242,7 @@ Implement **both** solutions for maximum compatibility:
    Should show the h-app div
 
 3. **Test with IndieAuth validator**:
-   Use https://indieauth.spec.indieweb.org/validator or a similar tool
+   Use https://www.w3.org/TR/indieauth/validator or a similar tool
 
 4. **Test actual auth flow**:
    - Navigate to /admin/login