Clean up admin-app dependencies and build configuration

[mojoatomic]

Context

PR #95 merged working admin app with temporary workarounds

Issues to fix:

1. React/react-dom installed at root - should be in admin-app package.json
2. Build artifacts (.js, .js.map) generated in src/ directories - update build config
3. ESM exports in package.json may not match actual build output
4. Jest tests failing with module resolution errors

Working reference:

• Tag: working-admin-app-with-workarounds
• Branch: fix/exports-esm
• Validation: scripts/validate-admin-data.mjs proves data pipeline works

Priority:

Non-blocking for demo. Admin UI functions correctly with workarounds.
Fix before public package release.