Remove dead code

[brookslybrand]

Remove Unused Code and Dependencies

Unused Files Removed (11)

• app/lib/observe-rect.ts – Rect observation utility (only used by deleted primitives)
• app/styles/docs.css – Unused docs styles
• app/styles/index.css – Unused index styles
• app/ui/primitives/ – Reach-style primitives:
  • collection.tsx, dropdown.tsx, menu-button.tsx, polymorphic.ts, popover.tsx, portal.tsx, rect.tsx, tabs.tsx

Dependencies Removed

• tabbable – Only used by removed popover.tsx
• @types/mdast – Unused type definitions

Unused Exports Made Internal

• app/lib/md.server.ts – getProcessor, loadPlugins, ProcessorOptions
• app/ui/buttons.tsx – Removed unused button link classes/components
• app/ui/primitives/utils.tsx – Internalized 17 helpers only used by removed primitives
• app/ui/subscribe.tsx – Removed SubscribeInput from exports
• server/redirects.ts – Internalized defaultRedirectsPath
• server/routes/blog-rss.ts – Internalized getBlogRssPosts and BlogRssPost
• app/lib/blog.server.ts – Internalized BlogAuthor and BlogPost
• server/middleware.ts – Internalized RateLimitOptions and FilteredLoggerOptions

Verification

• All tests pass (19 tests)
• TypeScript typecheck passes
• ESLint passes (1 existing warning in delegate-links.tsx)