fix(build-app): bail out if 'mktemp -d' fails instead of cp-ing into '/'

The DMG creation step sets DMG_TMP from 'mktemp -d' with no error check.
If mktemp fails (tmpfs full, permissions, TMPDIR misconfigured), DMG_TMP
is empty and the very next line — 'cp -a "\$APP_DIR" "\$DMG_TMP/"' —
expands to 'cp -a "<app>" "/"', which copies the bundle into the root of
the filesystem.

Refuse to continue unless mktemp produced a real directory. Defensive
second check catches the (rare) case where mktemp succeeds but returns
something that isn't a directory we can cp into.

scripts/build-app.sh

@@ -179,7 +179,11 @@ echo "  Creating DMG..."
 rm -f "$DMG_PATH"
 
 # Create a temporary directory for DMG contents
-DMG_TMP=$(mktemp -d)
+DMG_TMP=$(mktemp -d) || { echo "ERROR: mktemp -d failed — refusing to continue so we don't cp into the filesystem root." >&2; exit 1; }
+if [ -z "$DMG_TMP" ] || [ ! -d "$DMG_TMP" ]; then
+  echo "ERROR: mktemp -d returned an invalid path ('$DMG_TMP')." >&2
+  exit 1
+fi
 cp -a "$APP_DIR" "$DMG_TMP/"
 ln -s /Applications "$DMG_TMP/Applications"