feat(auth): DEBUG-Auto-Login (Memoro-Pattern)

Bei lokalen Xcode-Run-Builds wird beim Start automatisch eingeloggt
wenn der Keychain leer ist. Spart das manuelle Login bei jedem
Re-Install via Xcode.

- Sources/Core/Auth/DebugCredentials.swift — #if DEBUG-gewrappte
  Founder-Credentials (tills95@gmail.com / Aa-123456789)
- Sources/Core/Auth/AuthClient+EnsureSignedIn.swift — Extension
  ensureSignedIn() prüft .signedOut → signIn() in DEBUG
- RootView.task ruft auth.ensureSignedIn() — Release-Builds No-Op
  (Release/TestFlight/App-Store bleiben unverändert, User muss
  manuell einloggen)

Pattern 1:1 von memoro-native (gleiches File-Layout +
Klassennamen).

Build 9 → 10.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Sources/App/RootView.swift

@@ -9,15 +9,20 @@ struct RootView: View {
     @State private var pendingDeepLinkSlug: String?
 
     var body: some View {
-        switch auth.status {
-        case .signedIn:
-            mainTabs
-                .onOpenURL { url in handle(url: url) }
-                .onContinueUserActivity(NSUserActivityTypeBrowsingWeb) { activity in
-                    if let url = activity.webpageURL { handle(url: url) }
-                }
-        case .unknown, .signedOut, .signingIn, .error:
-            LoginView()
+        Group {
+            switch auth.status {
+            case .signedIn:
+                mainTabs
+                    .onOpenURL { url in handle(url: url) }
+                    .onContinueUserActivity(NSUserActivityTypeBrowsingWeb) { activity in
+                        if let url = activity.webpageURL { handle(url: url) }
+                    }
+            case .unknown, .signedOut, .signingIn, .error:
+                LoginView()
+            }
+        }
+        .task {
+            await auth.ensureSignedIn()
         }
     }