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/settings_actions.rs

@@ -79,6 +79,13 @@ impl ElyShell {
         }
     }
 
+    pub(crate) fn set_translucency_pct(&mut self, value: u8, cx: &mut Context<Self>) {
+        if let ShellState::Ready(core) = &mut self.state {
+            core.set_translucency_pct(value);
+            cx.notify();
+        }
+    }
+
     pub(super) fn reset_appearance(&mut self, cx: &mut Context<Self>) {
         if let ShellState::Ready(core) = &mut self.state {
             core.reset_appearance();