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.

FuelBoard/ContentView.swift

@@ -321,7 +321,7 @@ struct ContentView: View {
 #else
             let shouldSkipOnboarding = false
 #endif
-            if FuelStore.loadHasCompletedOnboarding()
+            if !FuelStore.shouldShowOnboarding()
                 || shouldSkipOnboarding {
                 locationManager.startForegroundTracking()
                 // Geofences and the Live Activity must follow the user even in