feat(create): migrate React templates to package import aliases

[tannerlinsley]

Summary

• migrate React template imports from @/ to package import aliases (#/) across base templates, add-ons, and examples
• add "imports": { "#/*": "./src/*" } to generated React app package.json
• keep compatibility for existing @/ imports by retaining tsconfig path mapping and vite-tsconfig-paths in generated React apps

Why

• moves template code toward package-import aliases while preserving compatibility with upstream scaffold files that still reference @/
• avoids breaking generated app builds during the transition

Validation

• pnpm --filter @tanstack/create test
• full pre-commit workspace build/test hooks
• smoke scaffold + install + build using local CLI: --add-ons ai,tRPC,tanstack-query

Fixes #218

Release note

• intentionally no changeset in this PR; batching release entries together as requested