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/update.rs

@@ -3582,6 +3582,14 @@ impl OpenCADStudio {
                 self.clean_screen ^= true;
                 Task::none()
             }
+            Message::ToggleTransparencyDisplay => {
+                let i = self.active_tab;
+                if i < self.tabs.len() {
+                    // No retessellate — the wire shader reads the flag from uniforms.
+                    self.tabs[i].scene.transparency_display ^= true;
+                }
+                Task::none()
+            }
             Message::ToggleUnitsPopup => {
                 self.units_popup_open ^= true;
                 Task::none()