Settings: move test notifications into hidden Debug section + dev flag

- Test-alert buttons moved out of the Alerts section into a Debug section.
- Debug section only renders when the debug flag is on; completely hidden
  otherwise (no user-facing toggle).
- Hidden toggle: tap Settings → About → Version five times to flip the
  flag (keychain + app-group storage, survives reinstall).
- About section added with version/build number.
- 35 tests pass.

Shared/FuelStore.swift

@@ -261,6 +261,7 @@ struct FuelStore {
     static let alertsEnabledKey = "fuelboard.alertsEnabled" // Bool
     static let alertsRadiusKey = "fuelboard.alertsRadius"   // Double km
     static let alertsFuelKey = "fuelboard.alertsFuel"       // FuelType raw value
+    static let debugModeKey = "fuelboard.debugMode"         // Bool — hidden dev flag
     static let onboardingCompletedKey = "fuelboard.onboardingCompleted" // Bool
     static let lastRefreshKey = "fuelboard.lastRefresh"      // TimeInterval (seconds since 1970)
 
@@ -401,6 +402,22 @@ struct FuelStore {
         saveString(unit.rawValue, service: distanceUnitKey)
     }
 
+    // MARK: Debug mode
+
+    /// Hidden developer flag. NOT exposed in the UI: toggled by tapping the
+    /// Settings → About → Version row five times. When on, the Settings tab
+    /// shows the Debug section (test notification buttons); when off, that
+    /// section is completely hidden from the UI. Stored keychain-first like
+    /// every other small setting so it survives app deletion during the
+    /// delete → reinstall test loop.
+    static func loadDebugMode() -> Bool {
+        loadString(service: debugModeKey) == "1"
+    }
+
+    static func saveDebugMode(_ enabled: Bool) {
+        saveString(enabled ? "1" : "0", service: debugModeKey)
+    }
+
     // MARK: Favourites
 
     /// A favourite pins a station FOR ONE fuel type. Starring a row while