SAP coordination: design catalog UI treatment for SAP-flagged datasets

### What
Design the user-facing display for dataset records in catalog.data.gov that are flagged as SAP-accessible. This includes a visible notice that the dataset is restricted but available through the Standard Application Process, and a button or link to the SAP application portal.

### Why
Once agencies are flagging datasets with the SAP indicator and the SAP portal is harvesting from Data.gov, public users discovering those datasets in catalog.data.gov need a clear path to apply for access. Without a UI treatment, the metadata flag has no user-facing value.

### Done When
- [ ] A design decision is documented covering: placement of the notice in the dataset record, button label, link destination, and display conditions
- [ ] Display logic is defined: notice and button render when `accessRights` indicates non-public AND the SAP flag is present on the record
- [ ] Sue has reviewed and approved the design approach before development begins
- [ ] Design accounts for the case where `accessRights` is non-public but no SAP flag is present (no button renders)

### Notes
- Initial implementation targets a static link to ResearchDataGov.org -- no pre-fill required for the pilot
- Deep-link and pre-filled application options are follow-on depending on SAP portal capabilities (to be confirmed with SAP PMO)
- Catalog is built on Python/Flask (datagov-catalog repo) -- relevant templates are in the repo
- DCAT-US 3.0 rollout is an upstream dependency; assume `accessRights` will be populated by the time this is implemented
- Do not render the button for datasets where `accessRights` is public, even if other access-related fields are present