Cleanup PageMeta

After adding new fields to PageMeta, several inconsistencies remain between the interface, its consumers, and the runtime data flow. This task addresses duplication, missing type connections, and stale patterns.

Issues

• description is defined in PageMeta but not wired through — pages still duplicate it in the Publication meta prop
• page-renderer.svelte.js reads pageMetadata as untyped Record<string, unknown> — should use PageMeta
• FeedItem in app.d.ts duplicates fields from PageMeta (tags, authors, cover) with slightly different types
• Publication.svelte MetaConfig typedef duplicates fields that could come from PageMeta
• PageModule interface (title, menuTitle) overlaps with PageMeta — consider removing it
• The cover field is typed as unknown in PageMeta but has a specific shape in FeedItem

Tasks

• Wire PageMeta.description through Publication so pages don't need to pass it twice
• Type pageMetadata in page-renderer.svelte.js as PageMeta
• Align FeedItem cover type with PageMeta cover type
• Consider deriving FeedItem fields from PageMeta using Pick
• Remove PageModule if fully superseded by PageMeta
• Add a shared cover image type to replace unknown
• Verify with pnpm check