refactor avatar display: unify rendering and resolve at use time

- Extract buildAvatarWidget() in common.dart to share avatar rendering
    logic across desktop settings, desktop CM and mobile CM
  - Add resolve_avatar_url() in Rust, exposed via FFI (SyncReturn),
    to resolve relative avatar paths (e.g. "/avatar/xxx") to absolute URLs
  - Store avatar as-is in local config, only resolve when displaying
    (settings page) or sending (LoginRequest)
  - Resolve avatar in LoginRequest before sending to remote peer
  - Add error handling for network image load failures
  - Guard against empty client.name[0] crash
  - Show avatar in mobile settings page account tile

Signed-off-by: 21pages <sunboeasy@gmail.com>

flutter/lib/mobile/pages/server_page.dart

@@ -1,5 +1,4 @@
 import 'dart:async';
-import 'dart:convert';
 
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
@@ -857,28 +856,17 @@ class ClientInfo extends StatelessWidget {
   }
 
   Widget _buildAvatar(BuildContext context) {
-    final avatar = client.avatar.trim();
-    if (avatar.isNotEmpty) {
-      if (avatar.startsWith('data:image/')) {
-        final comma = avatar.indexOf(',');
-        if (comma > 0) {
-          try {
-            return CircleAvatar(
-              backgroundImage: MemoryImage(base64Decode(avatar.substring(comma + 1))),
-            );
-          } catch (_) {}
-        }
-      } else if (avatar.startsWith('http://') || avatar.startsWith('https://')) {
-        return CircleAvatar(backgroundImage: NetworkImage(avatar));
-      }
-    }
-    // Show character as before if no avatar
-    return CircleAvatar(
+    final fallback = CircleAvatar(
       backgroundColor: str2color(
           client.name,
           Theme.of(context).brightness == Brightness.light ? 255 : 150),
-      child: Text(client.name[0]),
+      child: Text(client.name.isNotEmpty ? client.name[0] : '?'),
     );
+    return buildAvatarWidget(
+      avatar: client.avatar,
+      size: 40,
+      fallback: fallback,
+    )!;
   }
 }
 

flutter/lib/mobile/pages/settings_page.dart

@@ -689,7 +689,15 @@ class _SettingsState extends State<SettingsPage> with WidgetsBindingObserver {
                 title: Obx(() => Text(gFFI.userModel.userName.value.isEmpty
                     ? translate('Login')
                     : '${translate('Logout')} (${gFFI.userModel.accountLabelWithHandle})')),
-                leading: Icon(Icons.person),
+                leading: Obx(() {
+                  final avatar = bind.mainResolveAvatarUrl(
+                      avatar: gFFI.userModel.avatar.value);
+                  return buildAvatarWidget(
+                        avatar: avatar,
+                        size: 40,
+                      ) ??
+                      Icon(Icons.person);
+                }),
                 onPressed: (context) {
                   if (gFFI.userModel.userName.value.isEmpty) {
                     loginDialog();