Raise deployment target to iOS 18 and modernize SwiftUI APIs

Adopt modern SwiftUI patterns now that the minimum target is iOS 18:
NavigationStack, .toolbar, .tint, new Tab API with sidebarAdaptable
style, and remove iOS 17 availability checks. Add Liquid Glass effect
support for iOS 26 in TimerView and fix an optional interpolation
warning in AppState.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Nextcloud Cookbook iOS Client/Views/ReusableViews/LiquidGlassModifiers.swift

@@ -0,0 +1,28 @@
+//
+//  LiquidGlassModifiers.swift
+//  Nextcloud Cookbook iOS Client
+//
+//  Created by Vincent Meilinger on 14.02.26.
+//
+
+import SwiftUI
+
+struct TabBarMinimizeModifier: ViewModifier {
+    func body(content: Content) -> some View {
+        if #available(iOS 26, *) {
+            content.tabBarMinimizeBehavior(.onScrollDown)
+        } else {
+            content
+        }
+    }
+}
+
+struct BackgroundExtensionModifier: ViewModifier {
+    func body(content: Content) -> some View {
+        if #available(iOS 26, *) {
+            content.backgroundExtensionEffect()
+        } else {
+            content
+        }
+    }
+}