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,
   {

src/app/features/settings/general/General.tsx

@@ -327,6 +327,10 @@ function Appearance() {
   const [lotusTerminal, setLotusTerminal] = useSetting(settingsAtom, 'lotusTerminal');
   const [nightLightEnabled, setNightLightEnabled] = useSetting(settingsAtom, 'nightLightEnabled');
   const [nightLightOpacity, setNightLightOpacity] = useSetting(settingsAtom, 'nightLightOpacity');
+  const [glassmorphismSidebar, setGlassmorphismSidebar] = useSetting(
+    settingsAtom,
+    'glassmorphismSidebar',
+  );
 
   return (
     <Box direction="Column" gap="100">
@@ -431,6 +435,20 @@ function Appearance() {
         )}
       </SequenceCard>
 
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Glassmorphism Sidebar"
+          description="Semi-transparent sidebar with frosted glass effect. Works best with custom chat backgrounds."
+          after={
+            <Switch
+              variant="Primary"
+              value={glassmorphismSidebar}
+              onChange={setGlassmorphismSidebar}
+            />
+          }
+        />
+      </SequenceCard>
+
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile
           title="Lotus Terminal Mode"

src/app/pages/client/SidebarNav.tsx

@@ -1,5 +1,6 @@
 import React, { useRef } from 'react';
 import { Scroll } from 'folds';
+import classNames from 'classnames';
 
 import {
   Sidebar,
@@ -7,6 +8,7 @@ import {
   SidebarStackSeparator,
   SidebarStack,
 } from '../../components/sidebar';
+import { SidebarGlass } from '../../components/sidebar/Sidebar.css';
 import {
   DirectTab,
   HomeTab,
@@ -19,12 +21,15 @@ import {
   BookmarksTab,
 } from './sidebar';
 import { CreateTab } from './sidebar/CreateTab';
+import { useSetting } from '../../state/hooks/settings';
+import { settingsAtom } from '../../state/settings';
 
 export function SidebarNav() {
   const scrollRef = useRef<HTMLDivElement>(null) as React.RefObject<HTMLDivElement>;
+  const [glassmorphismSidebar] = useSetting(settingsAtom, 'glassmorphismSidebar');
 
   return (
-    <Sidebar>
+    <Sidebar className={classNames(glassmorphismSidebar && SidebarGlass)}>
       <SidebarContent
         scrollable={
           <Scroll ref={scrollRef} variant="Background" size="0">

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,