P2: Remove UI component module-level self-registration

[SisyphusZheng]

What to build

packages/ui/src/open-*.tsx files call customElements.define at module load. This complicates SSR, tests, and tree-shaking. Export classes without registering and provide an explicit registerOpenUi() function.

Acceptance criteria

• No UI module registers itself at import time.
• registerOpenUi() is exported from @openelement/ui.
• www and tests updated to call registration.

Blocked by

None