Add dark mode support with appearance picker and fix hardcoded colors

Add user-facing appearance setting (System/Light/Dark) wired via
preferredColorScheme at the app root. Replace hardcoded .black tints
and foreground styles with .primary so toolbar buttons and text remain
visible in dark mode. Remove profile picture from settings and
SwiftSoup from acknowledgements.

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

Nextcloud Cookbook iOS Client/Data/AppearanceMode.swift

@@ -0,0 +1,22 @@
+//
+//  AppearanceMode.swift
+//  Nextcloud Cookbook iOS Client
+//
+
+import Foundation
+
+enum AppearanceMode: String, CaseIterable {
+    case system = "system"
+    case light = "light"
+    case dark = "dark"
+
+    func descriptor() -> String {
+        switch self {
+        case .system: return String(localized: "System")
+        case .light: return String(localized: "Light")
+        case .dark: return String(localized: "Dark")
+        }
+    }
+
+    static let allValues: [AppearanceMode] = AppearanceMode.allCases
+}

Nextcloud Cookbook iOS Client/Data/UserSettings.swift

@@ -144,7 +144,13 @@ class UserSettings: ObservableObject {
             UserDefaults.standard.set(mealPlanSyncEnabled, forKey: "mealPlanSyncEnabled")
         }
     }
-    
+
+    @Published var appearanceMode: String {
+        didSet {
+            UserDefaults.standard.set(appearanceMode, forKey: "appearanceMode")
+        }
+    }
+
     init() {
         self.username = UserDefaults.standard.object(forKey: "username") as? String ?? ""
         self.token = UserDefaults.standard.object(forKey: "token") as? String ?? ""
@@ -168,7 +174,8 @@ class UserSettings: ObservableObject {
         self.remindersListIdentifier = UserDefaults.standard.object(forKey: "remindersListIdentifier") as? String ?? ""
         self.grocerySyncEnabled = UserDefaults.standard.object(forKey: "grocerySyncEnabled") as? Bool ?? true
         self.mealPlanSyncEnabled = UserDefaults.standard.object(forKey: "mealPlanSyncEnabled") as? Bool ?? true
-        
+        self.appearanceMode = UserDefaults.standard.object(forKey: "appearanceMode") as? String ?? AppearanceMode.system.rawValue
+
         if authString == "" {
             if token != "" && username != "" {
                 let loginString = "\(self.username):\(self.token)"