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]>

src/app/commands/mod.rs

@@ -754,6 +754,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "SKTOLERANCE",
         "CLIPROMPTLINES",
         "COMMANDLINEFADETIME",
+        "COMMANDLINEOPACITY",
         "COLORTHEME",
         "SELECTIONAREA",
         "SELECTIONAREAOPACITY",