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/render.rs

@@ -7,7 +7,7 @@ use gpui::{
 };
 
 use super::chrome::{
-    PANEL_BG, panel_shadow, render_command_overlay,
+    panel_bg, panel_shadow, render_command_overlay,
     render_topbar as render_topbar_chrome, render_wallpaper,
 };
 use super::sidebar::collapsed_sidebar_active;
@@ -86,6 +86,7 @@ impl ElyShell {
         sidebar_collapsed: bool,
         cx: &mut Context<Self>,
     ) -> AnyElement {
+        let panel_color = panel_bg(snapshot);
         div()
             .flex_1()
             .h_full()
@@ -93,7 +94,7 @@ impl ElyShell {
             .flex()
             .flex_col()
             .rounded(px(spacing::RADIUS_CARD))
-            .bg(rgba(PANEL_BG))
+            .bg(rgba(panel_color))
             .shadow(panel_shadow())
             .overflow_hidden()
             .child(render_topbar_chrome(self, snapshot, active_tab, sidebar_collapsed, cx))