Add translucency setting + apply to runtime panel alpha

Domain:
- AppearanceSettings gains translucency_pct (u8, 0..=100, default 40)
  with a clamping setter and serde round-trip coverage.
- DEFAULT_TRANSLUCENCY_PCT and MAX_TRANSLUCENCY_PCT exported for the
  shell.

Core:
- BrowserCore::set_translucency_pct delegates to the appearance struct;
  the existing reset_appearance covers the reset path.
- Integration test covers persistence into snapshot.appearance.

Render:
- chrome::sidebar::panel_bg(snapshot) replaces the static PANEL_BG
  constant, mapping the user's translucency_pct linearly into the alpha
  byte 0xff..0xb3. Sidebar (expanded + compact) and main pane consume
  the helper so changing the setting at runtime updates every glass
  surface in lock-step.

Form:
- Translucency row in chrome::appearance_form mirrors the design's
  static track + thumb visual driven by the persisted percentage, plus
  three preset chips (Solid 0 / Default 40 / Glassy 75) that mutate the
  setting through shell.set_translucency_pct.

Strict UX rule preserved: alpha never drops below 0xb3 so panels stay
readable without backdrop blur (which GPUI 0.2.2 doesn't expose).

crates/ely_app/src/shell/sidebar.rs

@@ -12,6 +12,7 @@ use gpui_component::{
     button::{Button, ButtonVariants},
 };
 
+use super::chrome::panel_bg;
 use super::{ElyShell, ShellState, render::tab_profile_label};
 use crate::ToggleSidebar;
 
@@ -31,6 +32,7 @@ impl ElyShell {
         sidebar_width: f32,
         cx: &mut Context<Self>,
     ) -> AnyElement {
+        let panel_color = panel_bg(snapshot);
         div()
             .w(px(sidebar_width))
             .h_full()
@@ -40,7 +42,7 @@ impl ElyShell {
             .gap_2()
             .p_2()
             .rounded(px(spacing::RADIUS_CARD))
-            .bg(rgba(PANEL_BG))
+            .bg(rgba(panel_color))
             .shadow(panel_shadow())
             .children(snapshot.favorites.iter().enumerate().map(|(index, tab)| {
                 self.render_compact_tab_button(
@@ -238,8 +240,6 @@ pub(super) fn collapsed_sidebar_active(sidebar_width: f32) -> bool {
     sidebar_width <= f32::from(COLLAPSED_SIDEBAR_WIDTH_PX)
 }
 
-const PANEL_BG: u32 = 0xffffffe0;
-
 fn panel_shadow() -> Vec<BoxShadow> {
     vec![
         BoxShadow {