feat: glassmorphism sidebar toggle (P5-3)

Settings → Appearance: "Glassmorphism Sidebar" toggle (off by default).
When enabled, applies backdrop-filter: blur(12px) and a semi-transparent
background to the left sidebar so chat background patterns show through.
SidebarGlass vanilla-extract class in Sidebar.css.ts; wired in
SidebarNav.tsx via classNames conditional.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/state/settings.ts

@@ -89,6 +89,8 @@ export interface Settings {
   nightLightEnabled: boolean;
   nightLightOpacity: number;
 
+  glassmorphismSidebar: boolean;
+
   deafenKey: string;
 
   warnOnUnverifiedDevices: boolean;
@@ -150,6 +152,8 @@ const defaultSettings: Settings = {
   nightLightEnabled: false,
   nightLightOpacity: 30,
 
+  glassmorphismSidebar: false,
+
   deafenKey: 'KeyM',
 
   warnOnUnverifiedDevices: false,