Add Settings tab: distance units toggle, onboarding replay, £4.99 tip; drop E10/E5 from labels

- FuelType labels: 'Unleaded (E10)'/'Premium (E5)' -> 'Unleaded'/'Premium'
- New DistanceUnit (miles/km) preference; all app + widget distances,
  radii and alert text convert/display in the chosen unit
- Settings tab (4th tab): segmented miles/km picker, 'Show introduction'
  replay button (onboarding moved out of Alerts tab), StoreKit tip £4.99
- ProximityMonitor + widget notifications use the chosen unit
- Tests: label expectations updated, DistanceUnit conversion + format tests

FuelBoardTests/Tests/FuelBoardSharedTests/FuelBoardTests.swift

@@ -196,10 +196,23 @@ final class BrandTests: XCTestCase {
 
 final class FuelTypeLabelTests: XCTestCase {
     func testDisplayNames() {
-        XCTAssertEqual(FuelType.e10.displayName, "Unleaded (E10)")
-        XCTAssertEqual(FuelType.e5.displayName, "Premium (E5)")
+        // E10/E5 grades are intentionally not part of user-facing labels.
+        XCTAssertEqual(FuelType.e10.displayName, "Unleaded")
+        XCTAssertEqual(FuelType.e5.displayName, "Premium")
         XCTAssertEqual(FuelType.diesel.displayName, "Diesel")
     }
+
+    func testDistanceUnitConversion() {
+        XCTAssertEqual(DistanceUnit.miles.toKM(1), 1.60934, accuracy: 0.00001)
+        XCTAssertEqual(DistanceUnit.kilometers.toKM(5), 5)
+        XCTAssertEqual(DistanceUnit.miles.fromKM(1.60934), 1, accuracy: 0.00001)
+        XCTAssertEqual(DistanceUnit.kilometers.fromKM(2.5), 2.5)
+    }
+
+    func testDistanceUnitFormat() {
+        XCTAssertEqual(DistanceUnit.miles.format(1.60934), "1.0 mi")
+        XCTAssertEqual(DistanceUnit.kilometers.format(3.4), "3.4 km")
+    }
 }
 
 final class FavouriteRefreshTests: XCTestCase {