Alerts choose their own fuel, independent of the Stations tab

The proximity monitor was tied to the Stations-tab fuel selection, so
switching the list to another fuel silently re-targeted geofences. Now the
Alerts tab has its own 'Fuel to monitor' picker (persisted as
fuelboard.alertsFuel):
- Monitor reads loadAlertsFuel() on init/background relaunch
- Changing the alerts fuel re-registers geofences around stations selling
  that fuel only
- Favourites priority slots stay scoped to the monitored fuel
- Tests: alerts fuel round-trip + default (33/33 passing)

FuelBoardTests/Tests/FuelBoardSharedTests/FuelBoardTests.swift

@@ -243,3 +243,18 @@ final class FavouriteRefreshTests: XCTestCase {
         XCTAssertTrue(diesel.id.hasPrefix("diesel|"))
     }
 }
+
+final class AlertsFuelTests: XCTestCase {
+    func testAlertsFuelRoundTrips() {
+        FuelStore.saveAlertsFuel(.diesel)
+        XCTAssertEqual(FuelStore.loadAlertsFuel(), .diesel, "alerts fuel persists independently")
+    }
+
+    func testAlertsFuelDefaultsToUnleaded() {
+        // Fresh state (test isolation) defaults to Unleaded like the app's
+        // other fuel selections.
+        FuelStore.saveAlertsFuel(.e5)
+        FuelStore.saveAlertsFuel(.e10)
+        XCTAssertEqual(FuelStore.loadAlertsFuel(), .e10)
+    }
+}