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/components/sidebar/Sidebar.css.ts

@@ -16,6 +16,13 @@ export const Sidebar = style([
   },
 ]);
 
+export const SidebarGlass = style({
+  backgroundColor: 'rgba(3, 5, 8, 0.55)',
+  backdropFilter: 'blur(12px)',
+  WebkitBackdropFilter: 'blur(12px)',
+  borderRight: '1px solid rgba(255,255,255,0.06)',
+});
+
 export const SidebarStack = style([
   DefaultReset,
   {