Fix accessible labels for Composer inputs

[Maya-Kid]

Why

Composer applied HTML attributes only to its outer div, so callers could not explicitly name its textarea or associate an external label with it.

What changes

Add a constrained textareaProps API for the input's id, aria-label, aria-labelledby, and aria-describedby, preserving wrapper attributes and the controlled input and keyboard behavior. Give the Composer examples and Agent Workspace input explicit message labels and document external label association.

How it was checked

• npm run docs, npm run typecheck, and npm run build pass on Node 26.3.0.
• Chrome 155, both themes: verified accessible input names, external label association, aria-labelledby precedence, descriptions, wrapper attributes, and the real Agent Workspace input.
• Rechecked composition/keyCode 229, Enter, Shift+Enter, canSubmit={false}, stop, Backspace-at-start, and multiline sizing. No browser errors.
• The complete examples gallery was checked in both themes during implementation: 389/389 mounted and 0 failed.

Before review

• Every commit is signed off (git commit -s)
• npm run typecheck and npm run build pass
• /dev/examples reports 0 failed
• What the change touches was looked at in both themes, open states included
• A component of Solid Glass's own has its example in src/examples/ and its page in content/solid/