Studio: audit native labels, disclosures and ad-hoc alerts against the primitives #2340

Part of #2201.

Summary

Audit three hand-rolled patterns under apps/web/src/features/ and move the ones that belong to a primitive:

• 1 native <label>: use the Label primitive.
• 6 aria-expanded disclosures: use Accordion or Collapsible where the element is a disclosure. Leave triggers that are already primitives.
• 19 ad-hoc role="alert" uses: use Alert where the element is a banner. Inline field errors stay as they are.

Done when

• The PR lists every call site with its decision: changed, or kept and why.
• Behaviour and accessible names are pinned first.
• Before | After | Prototype screenshots cover the changed surfaces.