onboarding: replay on a fresh/reinstalled install, not every launch

The keychain-first completed flag persists across reinstalls (same bundle id),
so a new install was treated as already-completed and onboarding silently
skipped while permissions still prompted. Add install identity:
- Store an install id in BOTH the app-own container defaults (wiped on
  reinstall, persists across launches) and keychain (survives reinstall).
- isFreshInstall() is true on first-ever launch or when the local id is
  missing/different from keychain (i.e. reinstalled); it seeds a fresh common
  id so the next launch of the same install is not 'fresh' again.
- shouldShowOnboarding() = not completed || fresh install -> onboarding now
  replays once per fresh/reinstall, never on ordinary launches.
Works on free SideStore (no app-group), using the app's own defaults domain
as the same-install signal.

Shared/FuelStore.swift

@@ -949,6 +949,43 @@ struct FuelStore {
         saveString(completed ? "1" : "0", service: onboardingCompletedKey)
     }
 
+    // MARK: Install identity — distinguish a genuinely fresh install (where
+    // onboarding should replay) from later launches of the same install.
+
+    /// Keychain copy of the install id (survives reinstall).
+    private static let installIDKey = "fuelboard.installID"
+    /// Local (app-own container) copy — wiped on reinstall, persists across
+    /// normal launches. Free SideStore accounts have no app-group container,
+    /// so this app-own defaults domain is the reliable "same install" signal.
+    private static let localInstallIDKey = "fuelboard.installID.local"
+
+    /// True when this is a first-ever install OR the app was just reinstalled
+    /// (local install id missing/different from the keychain id). Seeds a fresh
+    /// id into both stores so the next launch within the same install is not a
+    /// "fresh install" any more.
+    static func isFreshInstall() -> Bool {
+        let local = UserDefaults.standard.string(forKey: localInstallIDKey)
+        let remote = loadString(service: installIDKey)
+        if let local, let remote, local == remote { return false }
+        // Fresh / mismatched install (or first launch). Pattern a common id so
+        // subsequent launches of this install are recognised as the same one.
+        let id = UUID().uuidString
+        UserDefaults.standard.set(id, forKey: localInstallIDKey)
+        saveString(id, service: installIDKey)
+        return true
+    }
+
+    /// Onboarding should present when it hasn't been completed in this install
+    /// OR this is a freshly-installed app (so the walkthrough replays on a new
+    /// install, e.g. after a SideStore reinstall, without re-showing on every
+    /// ordinary launch). The install check always runs (it seeds the id) rather
+    /// than short-circuiting, so a brand-new install is recorded before the
+    /// user ever fills in onboarding.
+    static func shouldShowOnboarding() -> Bool {
+        let fresh = isFreshInstall()
+        return !loadHasCompletedOnboarding() || fresh
+    }
+
     // MARK: Road distances (Apple-Maps-matched, computed by the app)
 
     /// Cached road/routed distances (metres) keyed by station ID, computed by