Workspace + repo cleanup pass alongside two new public exports.

API additions:
- listReactiveContexts() — lists registered reactive contexts (for embeddr-debug)
- toast.warning() in EmbeddrAPI — non-fatal toast variant

Cleanup:
- Remove unused radix-ui umbrella dependency (only individual @radix-ui/* used)
- Delete dead vite.config.ts (library uses tsc, not vite — no app shell)
- Strip self-referential paths block from tsconfig.json; convert internal
  @embeddr/react-ui/* self-imports to relative paths
- Rename eslint.config.ts → eslint.config.js for consistency with sibling
  packages; switch to flat-config style (eslint, not eslint --ext)
- Fix broken prettier.config.cjs require path (was pointing at non-existent
  release/prettier.config.cjs); now correctly inherits workspace root config
- Drop boilerplate cruft from .gitignore (count.txt, .tanstack, .wrangler,
  .output, .vinxi, todos.json); add .github to .prettierignore (binary assets)
- Remove duplicate next-themes devDependency entry (kept dependency entry)
- Add typecheck script
- Run prettier across all sources at the new printWidth: 100, tabWidth: 2

Lint pass:
- @typescript-eslint/no-unnecessary-condition disabled — too aggressive for
  code that handles localStorage / parsed JSON where TS narrows to a literal
- Fix no-useless-escape errors in ArtifactContextMenu regex literals
- Remove dangling react-hooks/exhaustive-deps disable comment in
  usePanelLifecycle (rule already disabled at config level)