fix: keep the flag in front of a bare double dash

Discord's entry is `discord --url -- %u`. The flag went in before the
field code, which put it after the double dash, and that is where
Chromium stops reading switches and starts treating everything as an
address. The feature never came on.

src/lib/patch.sh

@@ -71,6 +71,11 @@ MCA_FEATURE_OPTS='--enable-blink-features|--enable-features'
 # Where an argument has to go: after the program and its own arguments, but
 # before the first field code (%U and friends) or Flatpak's @@ markers, which
 # are placeholders the launcher expands rather than arguments.
+#
+# A bare "--" stops it too. That is where Chromium stops reading switches and
+# starts treating everything as an address, so a flag put after it is read as a
+# page to open and the feature never comes on. Discord's entry is exactly that
+# shape: `discord --url -- %u`.
 _mca_exec_insert_at() {
 	local rest="$1" tok idx=0
 
@@ -79,7 +84,7 @@ _mca_exec_insert_at() {
 		[[ -n $rest ]] || break
 		tok="${rest%% *}"
 		case "$tok" in
-			%[a-zA-Z]|@@|@@u) printf '%s\n' "$idx"; return 0 ;;
+			%[a-zA-Z]|@@|@@u|--) printf '%s\n' "$idx"; return 0 ;;
 		esac
 		rest="${rest:${#tok}}"
 		idx=$(( idx + ${#tok} ))