Fix #36: Tab-Bar-Flicker durch UIKit/SwiftUI-Konflikt behoben

- SwiftUI .toolbarBackground/.toolbarColorScheme-Modifier für .tabBar
  aus ContentView entfernt – Konflikt mit UITabBar.appearance() beseitigt
- applyTabBarAppearance in statische Methode umgewandelt
- applyInitialTabBarAppearance() in AppDelegate.didFinishLaunchingWithOptions
  aufgerufen – setzt UIKit-Appearance VOR der View-Erstellung
- configureWithTransparentBackground → configureWithOpaqueBackground:
  verhindert weißes Bleed-through bei dunklen Themes
- Alpha-Komponente aus Tab-Bar-Hintergrundfarbe entfernt (vollständig opak)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nahbar/nahbar/ContentView.swift

@@ -52,11 +52,6 @@ struct ContentView: View {
                 .tabItem { Label("Einstellungen", systemImage: "gearshape") }
                 .tag(3)
         }
-        // Tab-Bar-Styling einmalig auf TabView statt auf jedem Tab setzen –
-        // verhindert weißes Aufblitzen beim Tab-Wechsel (Issue #36)
-        .toolbarBackground(theme.backgroundPrimary.opacity(0.92), for: .tabBar)
-        .toolbarBackground(.visible, for: .tabBar)
-        .toolbarColorScheme(theme.id.isDark ? .dark : .light, for: .tabBar)
         .fullScreenCover(isPresented: $showingNahbarOnboarding) {
             OnboardingContainerView {
                 nahbarOnboardingDone = true

nahbar/nahbar/NahbarApp.swift

@@ -16,6 +16,8 @@ final class AppDelegate: NSObject, UIApplicationDelegate, UNUserNotificationCent
         didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]? = nil
     ) -> Bool {
         UNUserNotificationCenter.current().delegate = self
+        // UIKit-Appearance VOR der View-Erstellung setzen – verhindert weißes Aufblitzen
+        NahbarApp.applyInitialTabBarAppearance()
         return true
     }
 
@@ -103,13 +105,13 @@ struct NahbarApp: App {
             .environment(\.nahbarTheme, activeTheme)
             .tint(activeTheme.accent)
             .onAppear {
-                applyTabBarAppearance(activeTheme)
+                NahbarApp.applyTabBarAppearance(activeTheme)
                 cloudSyncMonitor.startMonitoring(iCloudEnabled: icloudSyncEnabled)
                 AftermathNotificationManager.shared.registerCategory()
                 logger.info("App gestartet. Container-Modus: \(String(describing: NahbarApp.containerFallback))")
                 AppEventLog.shared.record("Container-Modus: \(NahbarApp.containerFallback)", level: .info, category: "Lifecycle")
             }
-            .onChange(of: activeThemeIDRaw) { _, _ in applyTabBarAppearance(activeTheme) }
+            .onChange(of: activeThemeIDRaw) { _, _ in NahbarApp.applyTabBarAppearance(activeTheme) }
             .onChange(of: icloudSyncEnabled) { _, enabled in
                 cloudSyncMonitor.startMonitoring(iCloudEnabled: enabled)
             }
@@ -122,11 +124,22 @@ struct NahbarApp: App {
         }
     }
 
-    private func applyTabBarAppearance(_ theme: NahbarTheme) {
-        let bg       = UIColor(theme.backgroundPrimary).withAlphaComponent(0.88)
+    /// Liest das aktive Theme aus UserDefaults und setzt UITabBar-Appearance
+    /// vor der ersten View-Erstellung (AppDelegate-Kontext).
+    static func applyInitialTabBarAppearance() {
+        let themeID = UserDefaults.standard.string(forKey: "activeThemeID")
+            .flatMap { ThemeID(rawValue: $0) } ?? .linen
+        applyTabBarAppearance(NahbarTheme.theme(for: themeID))
+    }
+
+    /// Setzt UITabBar- und UINavigationBar-Appearance für das gegebene Theme.
+    /// Statisch, damit AppDelegate und Theme-Change-Handler dieselbe Logik nutzen.
+    /// configureWithOpaqueBackground() verhindert das weiße Aufblitzen bei dunklen Themes.
+    static func applyTabBarAppearance(_ theme: NahbarTheme) {
+        let bg       = UIColor(theme.backgroundPrimary)  // vollständig opak – kein Bleed-through
         let normal   = UIColor(theme.contentTertiary)
         let selected = UIColor(theme.accent)
-        let border   = UIColor(theme.borderSubtle).withAlphaComponent(0.6)
+        let border   = UIColor(theme.borderSubtle)
 
         let item = UITabBarItemAppearance()
         item.normal.iconColor   = normal
@@ -135,7 +148,7 @@ struct NahbarApp: App {
         item.selected.titleTextAttributes = [.foregroundColor: selected]
 
         let tabAppearance = UITabBarAppearance()
-        tabAppearance.configureWithTransparentBackground()
+        tabAppearance.configureWithOpaqueBackground()  // statt transparent – stabil bei dunklen Themes
         tabAppearance.backgroundColor = bg
         tabAppearance.shadowColor = border
         tabAppearance.stackedLayoutAppearance       = item
@@ -145,11 +158,11 @@ struct NahbarApp: App {
         UITabBar.appearance().standardAppearance   = tabAppearance
         UITabBar.appearance().scrollEdgeAppearance = tabAppearance
 
-        let navBg       = UIColor(theme.backgroundPrimary).withAlphaComponent(0.92)
+        let navBg      = UIColor(theme.backgroundPrimary)
         let titleColor = UIColor(theme.contentPrimary)
 
         let navAppearance = UINavigationBarAppearance()
-        navAppearance.configureWithTransparentBackground()
+        navAppearance.configureWithOpaqueBackground()
         navAppearance.backgroundColor = navBg
         navAppearance.shadowColor = border
         navAppearance.titleTextAttributes      = [.foregroundColor: titleColor]