test: cover accessible component markup

[mgalore]

What changed?

Adds focused server-rendered accessibility assertions for the existing component macros: native button type and disabled state; labelled inputs and textareas with error descriptions; alert semantics; avatar alternative text; spinner status labels; and dialog labelling plus its keyboard-reachable close control.

Closes #5.

Checklist

• I ran make test.
• I ran make lint.
• I ran npm run build:css when changing templates or Tailwind styles.
• I added or updated tests where appropriate.
• I updated documentation for public API changes.
• I did not include secrets or generated files.

Screenshots

Not applicable: this change adds rendering assertions only and does not alter component markup.

[phcodesage]

Reviewed. This is a focused test-only change with no runtime or dependency impact. The assertions match the current component output and CI passes across Python 3.9–3.13, Tailwind, and GitGuardian. Approved.