SAP coordination: implement catalog UI notice and application link for SAP-flagged datasets

### What
Implement the approved UI treatment in catalog.data.gov so that dataset records with the SAP flag display a notice and a link to the SAP application portal.

### Why
This is the user-facing deliverable of the SAP coordination pilot -- the point where a researcher discovering a restricted dataset in Data.gov has a clear, actionable path to apply for access through the SAP.

### Done When
- [ ] Dataset record pages render a notice and application link when both conditions are met: `accessRights` is non-public AND the SAP flag is present
- [ ] No notice or button renders when either condition is absent
- [ ] Link destination is confirmed with Sue and the SAP PMO before deployment
- [ ] Change is tested against real or representative records with the SAP flag set
- [ ] PR reviewed and merged per standard split review process

### Notes
- Pilot link destination: static URL to ResearchDataGov.org (or a specific landing page if SAP PMO provides one)
- Follow-on options pending SAP PMO response:
- Option 2: deep link with dataset identifier in URL (e.g. `researchdatagov.org/apply?id=xyz`)
- Option 3: pre-filled application passing metadata fields via URL parameters
- If SAP PMO confirms support for option 2 or 3 before this ticket is built, update the implementation scope accordingly rather than building option 1 and retrofitting
- This ticket depends on the metadata flag definition ticket being complete
- DCAT-US 3.0 rollout is an upstream dependency