fix: make home dynamically render first pinned feed
- `/` now renders whatever feed is first in the pinned list
- Add dedicated `/feeds/recent` route so recent is accessible when not first
- Update FeedTabs, SidebarRight, MobileDrawer to link first pin to `/`
- Migrate old `path: "/"` to `/feeds/recent` for existing users
- Redirect to custom feed path if non-core feed is pinned first
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>