feat(statusbar): transparency-display toggle (TPY pill)

Add a transparency_enable uniform read by the wire shader: when the new
TPY pill is off, every line is forced opaque; when on, baked entity
transparency shows through. Toggling is a uniform write, no retessellate
(same mechanism as the lineweight-display toggle). Pill is toggleable
from the status-bar customization menu.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -750,6 +750,8 @@ pub enum Message {
     ToggleStatusPill(crate::ui::statusbar_config::StatusPill),
     /// Toggle clean-screen mode (hide ribbon + side panels).
     ToggleCleanScreen,
+    /// Toggle whether entity transparency is shown on screen.
+    ToggleTransparencyDisplay,
     /// Toggle the drawing-units picker open/closed.
     ToggleUnitsPopup,
     /// Close the drawing-units picker.