chore(relaunch): remove dead code orphaned by the redesign

The relaunch superseded a cluster of components that now have zero live
importers — removing them as a follow-up to the restyle:

- old shell/nav/search: components/Nav, components/Header (MinimalHeader),
  components/ui/Search, components/Layout/SidebarAppLayout
- legacy feed/content: components/Content (ContentCard), components/Feed
  AggregatedItem/VoteButtons/Composer/PopularTagsSidebar, components/PopularTags
- superseded by the new surfaces: components/Comments (→ DiscussionArea),
  components/ArticleSearch + components/ArticleMenu (→ ⌘K palette / new reader),
  PostEditor PostEditor.tsx + tabs/LinkTab + components/UrlMetadataPreview
  (route uses WriteTab; links go through the compose modal)
- app/(app)/articles/_client.tsx (the /articles route now redirects to /?type=article)

Barrels (Feed/index.ts, PostEditor/index.ts) pruned to the surviving exports.
tsc + eslint clean; / · /discussions · /jobs · /speakers · /volunteer all 200.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: NiallJoeMaher