Add post context to content-renderer sanitization warnings

Include author and permlink in renderer warning messages to help identify
which posts trigger blocked iframe/image warnings. Previously warnings like
"Blocked image (invalid src)" or "Blocked iframe (not whitelisted)" gave
no indication of which post content caused the issue.

Changes:
- Add PostContext interface with optional author/permlink fields
- Update TagTransformingSanitizer.sanitize() to accept PostContext
- Update warning messages to include context (e.g., "in @author/permlink")
- Export PostContext type from renderer package
- Update DefaultRenderer.render() to pass context through
- Update RendererContainer component to pass author/permlink
- Update content.tsx and comment-list-item.tsx call sites

Closes #763