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/chrome/mod.rs

@@ -21,7 +21,7 @@ pub(crate) use command_overlay::render_command_overlay;
 pub(crate) use home::render_home_page;
 pub(crate) use plugin_detail_view::render_plugin_detail_view;
 pub(crate) use settings_layout::render_settings_landing;
-pub(crate) use sidebar::{PANEL_BG, panel_shadow};
+pub(crate) use sidebar::{panel_bg, panel_shadow};
 pub(crate) use sidebar_header::render_sidebar_header;
 pub(crate) use split_pane::{
     pane_host_label, pane_path_label, pane_url_is_secure, render_compact_split_canvas,