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 <noreply@anthropic.com>

src/app/mod.rs

@@ -2607,6 +2607,8 @@ pub enum Message {
     ClipromptLinesChanged(i32),
     /// Change how long command-line history lines stay visible (COMMANDLINEFADETIME).
     CommandLineFadeChanged(i32),
+    /// Change the command line's panel opacity (COMMANDLINEOPACITY).
+    CommandLineOpacityChanged(u8),
     /// Toggle reversing the mouse-wheel zoom direction (ZOOMWHEEL).
     ZoomWheelReversedChanged(bool),
     TouchpadPanChanged(bool),