Fix/sensitive input UI

[gitsad]

What does this PR do?

Makes the sensitive-field (PII) marker in forms overridable. Previously the 🔒
badge next to a sensitive field's label was rendered directly by FormRenderer
with no override slot — unlike input/select/checkbox/textarea/file/submitButton —
so consumers couldn't restyle or remove it without a global CSS hack (which can't
be scoped per use-case and risks stripping PII markers from forms that legitimately
want them).

FormRenderer now resolves the badge through the existing element-override system
under a new sensitiveIndicator key. Consumers can swap in their own marker, or
return null to opt a given form scope out of the badge entirely — no CSS hacks,
fully scoped. Default rendering is unchanged (opt-out via override).

Also exports FormSensitiveIndicatorElementProps (the new override's props) and
FormFileElementProps (previously imported by the demo but not actually exported).
The demo wires up a custom PII chip indicator to demonstrate the slot.

Closes #

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing behavior to change)
• Refactor (no functional change)
• Documentation
• CI / tooling

Packages Affected

• @mobile-reality/mdma-spec
• @mobile-reality/mdma-parser
• @mobile-reality/mdma-runtime
• @mobile-reality/mdma-attachables-core
• @mobile-reality/mdma-renderer-react
• @mobile-reality/mdma-prompt-pack
• Blueprint: ___________

Checklist

• I have read the CONTRIBUTING guide.
• My code follows the existing code style (pnpm format and pnpm lint pass).
• I have added or updated tests that cover my changes.
• All tests pass (pnpm test).
• Type-checking passes (pnpm typecheck).
• I have added a changeset (pnpm changeset) if this change affects published packages.
• New or changed MDMA schemas are backwards-compatible (or marked as breaking).
• Sensitive fields are marked with sensitive: true where appropriate.

Note: no automated test added — the renderer-react package has no DOM test
harness (no react-dom / @testing-library/react), so the change is covered by
typecheck + manual verification in the demo. The "backwards-compatible schemas"
and "sensitive fields marked" items are N/A (no schema changes); checked as such.

How to Test

1. pnpm install && pnpm --filter @mobile-reality/mdma-renderer-react build
2. Run the demo and open a document containing a form with a sensitive: true
   field. With the demo customizations active, the default 🔒 badge is replaced by
   the custom PII chip (GlassSensitiveIndicator) — confirming the slot resolves.
3. Remove sensitiveIndicator from the demo's form.elements overrides → the
   default 🔒 badge renders, proving the default is unchanged.
4. Set it to sensitiveIndicator: () => null → the badge disappears entirely
   (the opt-out path consumers will use).