Improve page component + subcomponents overrides

[olliethedev]

Problem

Current plugin overrides are flat - components like PostCard or Link are defined at the plugin level. This makes it impossible to:

• Override subcomponents per-page (e.g., different PostCard for homepage vs tag page)
• Have typed component slot overrides for each page
• Compose pages from fully customizable subcomponent trees

Proposed Solution

Add structured page-level component overrides:

interface BlogPluginOverrides {
  // Existing flat overrides for shared components
  Link?: ComponentType<...>;
  PostCard?: ComponentType<...>;
  
  // New: Page-specific component slots
  pages?: {
    home?: {
      Header?: ComponentType<PageHeaderProps>;
      TagsList?: ComponentType<TagsListProps>;
      PostsList?: ComponentType<PostsListProps>;
    };
    post?: {
      Header?: ComponentType<...>;
      OnThisPage?: ComponentType<...>;
      PostNavigation?: ComponentType<...>;
    };
    // etc.
  };
}

This allows consumers to surgically replace subcomponents on specific pages while keeping defaults elsewhere.

Alternatives

• Compound component patterns with dot notation (BlogLayout.HomePage.Header)
• Render props for full control
• Context-based slot injection