Add BUILD.md and res/post-build-mac.sh

- BUILD.md: complete build instructions for macOS desktop (Xcode 26.5,
  Flutter 3.44, Dart 3.12, VCPKG, all patches explained)
- res/post-build-mac.sh: re-sign + install to /Applications + create dmg,
  run after every 'flutter build macos --release'

Key issues documented in BUILD.md:
  * Re-signing is mandatory (codesign --force --deep -s -)
  * Use hdiutil for dmg, not create-dmg (macos-alias is x86_64 broken)
  * libclang must be arm64 (use Xcode CLT, not homebrew llvm)
  * EXCLUDED_ARCHS=x86_64 for arm64-only Rust dylib

res/post-build-mac.sh

@@ -0,0 +1,93 @@
+#!/usr/bin/env bash
+# post-build-mac.sh
+# Re-sign and package a release .app into a .dmg for RustDesk (macOS).
+#
+# Must be run AFTER `flutter build macos --release` finishes.
+# The .app will NOT launch on macOS 26.5 without re-signing — see BUILD.md.
+#
+# Usage:
+#   ./res/post-build-mac.sh                # signs build output, installs to /Applications, creates dmg
+#   ./res/post-build-mac.sh --no-dmg       # just re-sign and install, skip dmg
+#   ./res/post-build-mac.sh --no-install   # just re-sign and create dmg in build dir
+#   VERSION=1.4.7 ./res/post-build-mac.sh  # custom version label in dmg filename
+
+set -euo pipefail
+
+# ---------- Config ----------
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+VERSION="${VERSION:-1.4.7}"
+ARCH="${ARCH:-arm64}"
+APP_REL="flutter/build/macos/Build/Products/Release/RustDesk.app"
+APP_PATH="$PROJECT_ROOT/$APP_REL"
+DMG_NAME="rustdesk-${VERSION}-${ARCH}.dmg"
+STAGING="/tmp/dmg_staging_postbuild"
+INSTALL_TARGET="${INSTALL_TARGET:-/Applications/RustDesk.app}"
+
+SKIP_DMG=0
+SKIP_INSTALL=0
+for arg in "$@"; do
+  case "$arg" in
+    --no-dmg) SKIP_DMG=1 ;;
+    --no-install) SKIP_INSTALL=1 ;;
+    -h|--help)
+      grep -E '^#( |!)' "$0" | sed 's/^# \{0,1\}//'
+      exit 0
+      ;;
+  esac
+done
+
+# ---------- Sanity checks ----------
+if [ ! -d "$APP_PATH" ]; then
+  echo "ERROR: $APP_PATH does not exist." >&2
+  echo "Run 'flutter build macos --release' first." >&2
+  exit 1
+fi
+
+# ---------- Step 1: Re-sign the .app ----------
+echo "==> Re-signing $APP_PATH"
+codesign --force --deep -s - "$APP_PATH"
+codesign --verify --strict --verbose=2 "$APP_PATH" 2>&1 | head -3 || true
+echo "    Signature: $(codesign -dvv "$APP_PATH" 2>&1 | grep -E '^Signature=' | head -1)"
+
+# ---------- Step 2: Install to /Applications ----------
+if [ "$SKIP_INSTALL" -eq 0 ]; then
+  echo "==> Installing to $INSTALL_TARGET"
+  pkill -9 -f "RustDesk.app/Contents/MacOS/RustDesk" 2>/dev/null || true
+  sleep 1
+  rm -rf "$INSTALL_TARGET" 2>/dev/null || true
+  cp -R "$APP_PATH" "$INSTALL_TARGET"
+  # Re-sign the installed copy (path is now different, so signatures may differ)
+  codesign --force --deep -s - "$INSTALL_TARGET"
+  echo "    Installed: $INSTALL_TARGET"
+fi
+
+# ---------- Step 3: Build the .dmg ----------
+if [ "$SKIP_DMG" -eq 0 ]; then
+  echo "==> Building $DMG_NAME"
+  rm -rf "$STAGING"
+  mkdir -p "$STAGING"
+  cp -R "$APP_PATH" "$STAGING/RustDesk.app"
+  ln -sf /Applications "$STAGING/Applications"
+  # CRITICAL: re-sign the staging copy too — hdiutil snapshots whatever's there
+  codesign --force --deep -s - "$STAGING/RustDesk.app"
+
+  DMG_OUT="$PROJECT_ROOT/$DMG_NAME"
+  rm -f "$DMG_OUT"
+  hdiutil create -volname "RustDesk Installer" \
+    -srcfolder "$STAGING" \
+    -ov -format UDZO \
+    "$DMG_OUT"
+  echo "    DMG: $DMG_OUT ($(du -h "$DMG_OUT" | awk '{print $1}'))"
+fi
+
+# ---------- Step 4: Cleanup ----------
+rm -rf "$STAGING"
+
+echo ""
+echo "Done. To launch:"
+echo "  open $INSTALL_TARGET"
+if [ "$SKIP_DMG" -eq 0 ]; then
+  echo "To distribute:"
+  echo "  open $PROJECT_ROOT/$DMG_NAME"
+fi