ASA Go: Geolocation selected fire zone/centre

**Describe the task**
On first load, no Centre is chosen (default on desktop app) and the province fits in the height. The Message bar at the top of screen should say: "Click a Fire Centre to get started."
Nice to have: on mobile, if we have access to the user's geolocation, the Fire Centre they're in should be the fallback.

**Acceptance Criteria**
- [ ] first
- [ ] second
- [ ] third

**Additional context**
- Add any other context about the task here.
- Or here

Start by locating the ASA Go screen and its Message bar implementation, then review how Fire Centre selection and mobile geolocation are currently handled. Done should include the specified first-load message and a defined mobile fallback, but the placeholder acceptance criteria need clarification before implementation.

Written by the indexing model from the issue text.