feat(settings): shared save path for preference toggles

savePreference(_:) reads the current OutlineUserPreferences, flips one
field, sends the whole object via updateUserPreferences. Every Behavior/
Display toggle added next reuses this instead of its own copy of the
same read-modify-write.

Outpost/Features/Account/SettingsView.swift

@@ -34,6 +34,8 @@ struct SettingsView: View {
     @State private var nameErrorMessage: String?
     @State private var isSavingLanguage = false
     @State private var languageErrorMessage: String?
+    @State private var isSavingPreferences = false
+    @State private var preferencesErrorMessage: String?
 
     /// Full Local Sync and cache-clearing both need a real connection to be
     /// safe — clearing while offline (or letting Full Local Sync think it
@@ -248,6 +250,12 @@ struct SettingsView: View {
                 Divider()
                 appearanceRow
             }
+
+            if let preferencesErrorMessage {
+                Text(preferencesErrorMessage)
+                    .font(.caption)
+                    .foregroundStyle(.red)
+            }
         }
         .frame(maxWidth: 480, alignment: .leading)
         .task { await refreshProfile() }
@@ -338,6 +346,46 @@ struct SettingsView: View {
         }
     }
 
+    /// Every toggle in the Preferences page shares this: read
+    /// `session.userPreferences` (or an empty object if nothing's been set
+    /// yet), flip the one field the caller cares about, send the *whole*
+    /// object back — see `UpdateUserPreferencesRequest`'s own doc comment
+    /// for why the full object rather than a partial diff.
+    private func savePreference(_ update: (inout OutlineUserPreferences) -> Void) async {
+        guard let apiClient = session.apiClient, let userId = session.userId else { return }
+        var preferences = session.userPreferences ?? OutlineUserPreferences()
+        update(&preferences)
+        isSavingPreferences = true
+        defer { isSavingPreferences = false }
+        do {
+            let updated = try await apiClient.updateUserPreferences(
+                UpdateUserPreferencesRequest(id: userId, preferences: preferences)
+            )
+            session.applyUpdatedProfile(updated)
+            preferencesErrorMessage = nil
+        } catch {
+            preferencesErrorMessage = outlineErrorMessage(error, fallback: "Couldn't update your preferences.")
+        }
+    }
+
+    private func preferenceToggleRow(
+        _ title: String,
+        description: String,
+        isOn: Bool,
+        onChange: @escaping (Bool) -> Void
+    ) -> some View {
+        VStack(alignment: .leading, spacing: 2) {
+            Toggle(isOn: Binding(get: { isOn }, set: onChange)) {
+                VStack(alignment: .leading, spacing: 2) {
+                    Text(title)
+                    Text(description)
+                        .font(.caption)
+                        .foregroundStyle(.secondary)
+                }
+            }
+        }
+    }
+
     // MARK: - Offline & Sync
 
     private var offlineSyncDetail: some View {