feat(settings): Preferences appearance setting

Reuses the existing local Appearance color-scheme picker instead of a
second divergent implementation — this has always been a device-side
preference in this app, not a server-synced one.

Outpost/Features/Account/SettingsView.swift

@@ -245,6 +245,8 @@ struct SettingsView: View {
 
             preferencesSubsection("Display") {
                 languageRow
+                Divider()
+                appearanceRow
             }
         }
         .frame(maxWidth: 480, alignment: .leading)
@@ -292,6 +294,30 @@ struct SettingsView: View {
         }
     }
 
+    /// Same `@AppStorage("outpost.appearance")` binding the standalone
+    /// Appearance section already uses — this is a local-only, device-side
+    /// preference (matches how this app has always handled it), not a
+    /// server-synced Outline preference, so it doesn't need its own
+    /// save call or error state.
+    private var appearanceRow: some View {
+        HStack {
+            VStack(alignment: .leading, spacing: 2) {
+                Text("Appearance")
+                Text("Choose your preferred interface colour scheme.")
+                    .font(.caption)
+                    .foregroundStyle(.secondary)
+            }
+            Spacer()
+            Picker("Appearance", selection: $appearance) {
+                ForEach(AppAppearance.allCases) { option in
+                    Text(option.label).tag(option)
+                }
+            }
+            .labelsHidden()
+            .frame(width: 200)
+        }
+    }
+
     private var languageBinding: Binding<String> {
         Binding(
             get: { session.userLanguage ?? "en_US" },