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/projectplan/v1/implementation-plan.md

@@ -1573,7 +1573,7 @@ Final steps before V1 release.
 
 ### External Standards
 - [Micropub Specification](https://micropub.spec.indieweb.org/)
-- [IndieAuth Specification](https://indieauth.spec.indieweb.org/)
+- [IndieAuth Specification](https://www.w3.org/TR/indieauth/)
 - [Microformats2](http://microformats.org/wiki/microformats2)
 - [RSS 2.0 Specification](https://www.rssboard.org/rss-specification)
 - [IndieLogin API](https://indielogin.com/api)