desktop: replace copy-assets shell script with gradle task (#6900)

scripts/android/copy-assets.sh

@@ -1,21 +0,0 @@
-#!/bin/sh
-set -eu
-
-# Copies generated multiplatform assets into the SimpleX assets directory.
-# Called by Gradle build when simplex.assets.dir property is set.
-#
-# Usage: copy-assets.sh <source-dir> <dest-dir>
-
-SRC_DIR="$1/multiplatform/resources/MR/images"
-DEST_DIR="$2/MR/images"
-
-if [ ! -d "$SRC_DIR" ]; then
-  echo "Error: source assets not found: $SRC_DIR (run resize.sh first)" >&2
-  exit 1
-fi
-
-rm -rf "$DEST_DIR"
-mkdir -p "$DEST_DIR"
-
-cp "$SRC_DIR"/* "$DEST_DIR/"
-echo "Copied multiplatform assets to $DEST_DIR"