Raster images: transparency off draws pixels opaque; TRANSPARENCY command

An image's transparency flag decides whether transparent pixels show what
is behind the image or draw in their stored colour: the image pass and PDF
plots now follow it (it was ignored, so every PNG alpha showed through).
TRANSPARENCY selects images and asks for the mode, defaulting to the first
image's current one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/locale_catalog.rs

@@ -1197,6 +1197,9 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "Frames & Print" => Some(("insert", "frames-print")),
         "Frames Off" => Some(("insert", "frames-off")),
         "Frames On" => Some(("insert", "frames-on")),
+        "Select image(s):" => Some(("insert", "select-images")),
+        "Enter transparency mode [ON/OFF] <ON>:" => Some(("insert", "transparency-mode-on")),
+        "Enter transparency mode [ON/OFF] <OFF>:" => Some(("insert", "transparency-mode-off")),
         "Frames On, Not Plotted" => Some(("draw", "frames-on-not-plotted")),
         "Freeze" => Some(("layers", "freeze")),
         "From DXF" => Some(("properties", "from-dxf")),