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_browser_core/tests/appearance.rs

@@ -21,11 +21,13 @@ fn setters_persist_into_subsequent_snapshots() -> Result<(), Box<dyn Error>> {
     core.set_wallpaper_theme(WallpaperTheme::Mint);
     core.set_theme_mode(ThemeMode::Dark);
     core.set_reduce_motion(true);
+    core.set_translucency_pct(75);
 
     let snapshot = core.snapshot()?;
     assert_eq!(snapshot.appearance.wallpaper(), WallpaperTheme::Mint);
     assert_eq!(snapshot.appearance.theme_mode(), ThemeMode::Dark);
     assert!(snapshot.appearance.reduce_motion());
+    assert_eq!(snapshot.appearance.translucency_pct(), 75);
     Ok(())
 }