feat: signed customization client-side hooks

Companion to the rustdesk-server Customization admin page that
produces signed custom.txt blobs.

- src/common.rs: replaces the hardcoded branding pubkey with a
  build-time RUSTDESK_BRANDING_PUBKEY env (option_env!), falling
  back to the operator's primary pubkey so unattended builds still
  produce a working client. Per-customer keys can be baked in via
  CI without source edits.
- flutter/lib/common.dart: adds getAppIconBytes() (cached, base64
  decoded once) and patches loadLogo / loadIcon to honor the buildin
  app-icon. This covers every existing call-site — desktop home
  page, server page, tabbar, and mobile settings — without touching
  any of them.

OPTION_APP_ICON itself ships in the hbb_common submodule (already
bumped in the previous commit).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter/lib/common.dart

@@ -3702,8 +3702,35 @@ Widget loadPowered(BuildContext context) {
   ).marginOnly(top: 6);
 }
 
+// Cached decoded `app-icon` bytes from the signed custom.txt blob. The blob
+// stores the image as standard base64 under the buildin key "app-icon"; we
+// decode once on first read and reuse the bytes for every Image.memory call
+// below. Empty Uint8List means "no custom icon" and we fall through to the
+// bundled assets.
+Uint8List? _appIconBytes;
+bool _appIconChecked = false;
+Uint8List? getAppIconBytes() {
+  if (_appIconChecked) return _appIconBytes;
+  _appIconChecked = true;
+  final b64 = bind.mainGetBuildinOption(key: 'app-icon');
+  if (b64.isEmpty) return null;
+  try {
+    _appIconBytes = base64.decode(b64);
+  } catch (_) {
+    _appIconBytes = null;
+  }
+  return _appIconBytes;
+}
+
 // max 300 x 60
 Widget loadLogo() {
+  final custom = getAppIconBytes();
+  if (custom != null) {
+    return Container(
+      constraints: BoxConstraints(maxWidth: 300, maxHeight: 60),
+      child: Image.memory(custom, fit: BoxFit.contain),
+    ).marginOnly(left: 12, right: 12, top: 12);
+  }
   return FutureBuilder<ByteData>(
       future: rootBundle.load('assets/logo.png'),
       builder: (BuildContext context, AsyncSnapshot<ByteData> snapshot) {
@@ -3725,6 +3752,10 @@ Widget loadLogo() {
 }
 
 Widget loadIcon(double size) {
+  final custom = getAppIconBytes();
+  if (custom != null) {
+    return Image.memory(custom, width: size, height: size, fit: BoxFit.contain);
+  }
   return Image.asset('assets/icon.png',
       width: size,
       height: size,