feat(ai-studio): warn when every decision field is Hidden

[jimmeryn]

What changed

When the proposal source declares fields but the author leaves every one Hidden, the Human task panel shows a warning below the field rows:

No field is visible to the decider. Set at least one field to Read-only or Editable.

• The warning disappears as soon as one field is Read-only, Editable or Required.
• It shows at once on a Human task fresh from the palette connected to an AI Agent with a structured Response format, because that node stores an empty schema.
• It never shows together with the source hints (unconnected, ambiguous, noFields). If the source stops declaring fields, noFields wins.
• A field stored in the schema but not declared by the source never triggers it.
• The README describes the panel's live regions.

Decisions

• Frontend only. Run is not blocked, and the copy promises nothing about Run, because the backend accepts an empty decision form today.
• The warning sits below the rows, inside a role="status" region that stays mounted. Screen readers often skip a live region that is mounted together with its text.
• The rule is a pure function (isEveryFieldHidden) kept apart from the source hints, because it renders in a different place and is a live region.

Open questions

• Should the backend refuse an empty decisionRequest.schema.properties when Run starts? If it does, the copy can say that Run flags the block, as the prototype does.
• A Run refusal currently shows only the generic snackbar. details[].domainCode is not read, so the author does not learn which node or rule failed.

Verified

• AI Studio tests, typecheck, lint.
• Manually: the warning appears and clears, the Plain text switch, a fresh node from the palette, the Run view, light and dark. Not checked with a screen reader.