feat(settings): Passkeys section

Informational only, per Outline itself — passkey/WebAuthn registration
needs a browser context, so this stays web-only rather than a
placeholder for missing native functionality.

Outpost/Features/Account/SettingsView.swift

@@ -82,6 +82,7 @@ struct SettingsView: View {
         case .profile: profileDetail
         case .preferences: preferencesDetail
         case .notifications: notificationsDetail
+        case .passkeys: passkeysDetail
         case .offlineSync: offlineSyncDetail
         case .advanced: advancedDetail
         case .about: aboutDetail
@@ -715,6 +716,29 @@ struct SettingsView: View {
         }
     }
 
+    // MARK: - Passkeys
+
+    /// Read-only on purpose — Outline only exposes passkey management from
+    /// its own web app (WebAuthn registration needs a browser context this
+    /// native app doesn't have), so this page is informational, not a
+    /// placeholder for missing functionality.
+    private var passkeysDetail: some View {
+        VStack(alignment: .leading, spacing: 16) {
+            sectionHeader
+            Text("Passkeys allow you to sign in safely without a password using your device's biometric authentication (Face ID, Touch ID, Windows Hello) or security key.")
+                .font(.subheadline)
+                .foregroundStyle(.secondary)
+                .frame(maxWidth: 480, alignment: .leading)
+
+            Label("This setting can only be changed from the web version of Outline.", systemImage: "lock.fill")
+                .font(.callout)
+                .foregroundStyle(.secondary)
+                .padding(12)
+                .frame(maxWidth: 480, alignment: .leading)
+                .background(.quaternary.opacity(0.5), in: RoundedRectangle(cornerRadius: 8, style: .continuous))
+        }
+    }
+
     // MARK: - Offline & Sync
 
     private var offlineSyncDetail: some View {