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/pick_intent_service.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/services.dart';
 import '../models/pick_request.dart';
+import 'native_channel.dart';
 
 /// Talks to `MainActivity.kt`'s hand-rolled GET_CONTENT picker handling
 /// (mirrors `ShareIntentService`'s split between a cold-start check and a
@@ -14,7 +15,8 @@ class PickIntentService {
   /// Non-null if the app was launched cold as another app's file/photo
   /// picker.
   static Future<PickRequest?> getPickRequest() async {
-    final result = await _methodChannel.invokeMethod<Map<dynamic, dynamic>>(
+    final result = await invokeIfAvailable<Map<dynamic, dynamic>>(
+      _methodChannel,
       'getPickRequest',
     );
     return result == null ? null : PickRequest.fromMap(result);
@@ -23,7 +25,8 @@ class PickIntentService {
   /// Emits whenever the app is asked to act as a picker while already
   /// running.
   static Stream<PickRequest> get onNewPickRequest {
-    return _newPickChannel.receiveBroadcastStream().map(
+    return quietEvents(
+      _newPickChannel,
       (event) => PickRequest.fromMap(event as Map<dynamic, dynamic>),
     );
   }
@@ -34,7 +37,7 @@ class PickIntentService {
     List<String> localPaths,
     List<String> mimeTypes,
   ) {
-    return _methodChannel.invokeMethod('finishPick', {
+    return invokeIfAvailable(_methodChannel, 'finishPick', {
       'paths': localPaths,
       'mimeTypes': mimeTypes,
     });
@@ -42,6 +45,6 @@ class PickIntentService {
 
   /// Backs out of picking mode with no result, closing the picker.
   static Future<void> cancelPick() {
-    return _methodChannel.invokeMethod('cancelPick');
+    return invokeIfAvailable(_methodChannel, 'cancelPick');
   }
 }