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/models/user_model.dart

@@ -34,6 +34,7 @@ class UserModel {
     }
     return '$preferred (@$username)';
   }
+
   WeakReference<FFI> parent;
 
   UserModel(this.parent) {
@@ -88,7 +89,6 @@ class UserModel {
       }
 
       final user = UserPayload.fromJson(data);
-      user.avatar = _resolveAvatar(user.avatar, url);
       _parseAndUpdateUser(user);
     } catch (e) {
       debugPrint('Failed to refreshCurrentUser: $e');
@@ -138,7 +138,6 @@ class UserModel {
     avatar.value = user.avatar;
     isAdmin.value = user.isAdmin;
     bind.mainSetLocalOption(key: 'user_info', value: jsonEncode(user));
-    _updateLocalUserInfo();
     if (isWeb) {
       // ugly here, tmp solution
       bind.mainSetLocalOption(key: 'verifier', value: user.verifier ?? '');