fix(z-input): add aria-describedby for password fields

[ada-workbackai]

Summary

Fixes WCAG 3.3.2 (Labels or Instructions) Level A by adding programmatic association between password input fields and their requirement instructions via aria-describedby.

Issue: Password fields in registration forms lack instructions about password requirements (minimum length, character types, etc.). Screen reader users cannot access these requirements if they are not programmatically associated with the input field, leading to form abandonment and accessibility violations.

Solution: Modified the z-input component to automatically add aria-describedby attribute when a message prop is provided, linking the input to the helper text element.

Changes

• Added aria-describedby attribute to input elements when message prop is set
• Added id attribute (${htmlid}_message) to z-input-message component
• Added password example story demonstrating proper usage with requirements text
• Fixed label class name for consistency (z-label → input-label)

Test Plan

• Added story example showing password field with requirements message
• Verified aria-describedby correctly references message element ID
• Lint checks pass
• Message is programmatically associated and announced by screen readers

Usage Example

<z-input
  type="password"
  label="Password"
  placeholder="Inserisci la password"
  required={true}
  minlength={8}
  message="La password deve contenere almeno 8 caratteri"
/>

This will render an input with aria-describedby="input-id_message" referencing the message element, ensuring screen readers announce the requirements when the field receives focus.

Evidence

View before/after screenshots and full audit details:
https://app.workback.ai/dashboard/issue/5036/

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WCAG Reference:
3.3.2 Labels or Instructions (Level A)