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/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"