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/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();