feat(command-line): adjustable command line opacity

COMMANDLINEOPACITY (20-100 percent, default 100) and a slider under
Options > Display make the floating command line's panels see-through,
so the drawing stays visible behind it. The input field stays opaque
for legibility. The value is an application setting that persists.

Refs #1494

Co-Authored-By: Claude Opus 5.5 <[email protected]>

locales/ru-RU/opencadstudio.ftl

@@ -285,6 +285,7 @@ command-copy =
     .mode = COPY  Задайте режим копирования [Single/Multiple] <Multiple>:
 
 common =
+    .command-line-opacity = Непрозрачность командной строки
     .txtexp-no-text-selected = TXTEXP: текст не выбран.
     .angle-override-shown = Переопределение угла: __ocs_fmt_0__
     .plot-preview-task = Предварительный просмотр печати