feat: resolve open component issues

[secrethacker548-pixel]

Summary

• Add the native select_field macro with selected/disabled options and accessible help/error states.
• Add semantic CSS tokens with opt-in data-theme="dark" support and a demo toggle.
• Document every component with copy-paste Jinja examples and expand the local gallery.
• Add an optional HTMX demo with a server-rendered form fallback.
• Add Playwright desktop, mobile, and dark-theme visual snapshots with CI coverage.
• Update rendering tests, package metadata, the reverse-engineering notebook, and contributor instructions.
• Merge the latest target main and preserve its accessibility coverage.

Verification

• .venv/bin/python -m pytest -q — 10 passed
• .venv/bin/ruff check . — passed
• npm run build:css — passed
• .venv/bin/python -m build --wheel --sdist — passed
• Playwright visual suite — 6 passed locally

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Closes #6