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/common.dart

@@ -4118,3 +4118,43 @@ String mouseButtonsToPeer(int buttons) {
       return '';
   }
 }
+
+/// Build an avatar widget from an avatar URL or data URI string.
+/// Returns [fallback] if avatar is empty or cannot be decoded.
+/// [borderRadius] defaults to [size]/2 (circle).
+Widget? buildAvatarWidget({
+  required String avatar,
+  required double size,
+  double? borderRadius,
+  Widget? fallback,
+}) {
+  final trimmed = avatar.trim();
+  if (trimmed.isEmpty) return fallback;
+
+  ImageProvider? imageProvider;
+  if (trimmed.startsWith('data:image/')) {
+    final comma = trimmed.indexOf(',');
+    if (comma > 0) {
+      try {
+        imageProvider = MemoryImage(base64Decode(trimmed.substring(comma + 1)));
+      } catch (_) {}
+    }
+  } else if (trimmed.startsWith('http://') || trimmed.startsWith('https://')) {
+    imageProvider = NetworkImage(trimmed);
+  }
+
+  if (imageProvider == null) return fallback;
+
+  final radius = borderRadius ?? size / 2;
+  return ClipRRect(
+    borderRadius: BorderRadius.circular(radius),
+    child: Image(
+      image: imageProvider,
+      width: size,
+      height: size,
+      fit: BoxFit.cover,
+      errorBuilder: (_, __, ___) =>
+          fallback ?? SizedBox.shrink(),
+    ),
+  );
+}