iOS: degrade gracefully where the Android-only native services are missing

Share/pick/sync calls and streams no longer throw MissingPluginException at
startup; upload/download/sync-now show 'X isn't available on this platform
yet'. Offline/sync mirror dir no longer uses the Android-only external dir.
Info.plist: Face ID and photo library usage strings.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

lib/services/native_channel.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/services.dart';
 import '../providers/session_controller.dart';
 
 /// The "get this account's auth header, or bail" guard every native-
@@ -21,3 +22,57 @@ Map<String, String> baseChannelArgs(SessionController session) {
     'authHeader': requireAuthHeader(session),
   };
 }
+
+/// Thrown by user-triggered native services (upload, download, sync now...)
+/// on a platform whose native side isn't implemented yet - the message is
+/// shown to the user as-is, e.g. in a snackbar.
+class NativeServiceUnavailable implements Exception {
+  final String feature;
+
+  const NativeServiceUnavailable(this.feature);
+
+  @override
+  String toString() => "$feature isn't available on this platform yet.";
+}
+
+/// Calls [method], treating a missing native implementation (no
+/// `MethodChannel` handler registered, as on iOS until each service is
+/// built there) as "nothing to report": returns null instead of throwing
+/// `MissingPluginException`. For cold-start checks, scheduling and other
+/// calls nobody is waiting on.
+Future<T?> invokeIfAvailable<T>(
+  MethodChannel channel,
+  String method, [
+  Object? arguments,
+]) async {
+  try {
+    return await channel.invokeMethod<T>(method, arguments);
+  } on MissingPluginException {
+    return null;
+  }
+}
+
+/// Like [invokeIfAvailable], but for an action the user just asked for:
+/// throws [NativeServiceUnavailable] naming [feature] rather than silently
+/// doing nothing.
+Future<T?> invokeOrExplain<T>(
+  MethodChannel channel,
+  String method,
+  String feature, [
+  Object? arguments,
+]) async {
+  try {
+    return await channel.invokeMethod<T>(method, arguments);
+  } on MissingPluginException {
+    throw NativeServiceUnavailable(feature);
+  }
+}
+
+/// [channel]'s broadcast stream mapped through [convert], with the
+/// "no native implementation" error dropped - the stream just never emits.
+Stream<T> quietEvents<T>(EventChannel channel, T Function(dynamic) convert) {
+  return channel
+      .receiveBroadcastStream()
+      .handleError((Object _) {}, test: (e) => e is MissingPluginException)
+      .map(convert);
+}