ASA Go: Implement Loading & Error States

**Describe the task**

Create Error Messaging + Proposals to resolve

**Acceptance Criteria**
- [ ] Layer Loading in progress
- [ ] Layer Loading Failure
- [ ] PMTiles Loading Failure
- [ ] API failure

**Additional context**
- #4799
- [Designs on 4701](https://github.com/bcgov/wps/issues/4701)

Start by locating the ASA Go layer-loading and API error handling entry points, then review the designs linked from issue #4701 and the context in #4799. Trace how layer and PMTiles requests report loading and failure states. Done means all four acceptance criteria have visible, consistent states, with tests or manual checks covering each failure path.

Written by the indexing model from the issue text.