feat(02-02): brand sidebar with wordmark, active indicator, and collapse trigger

- Add SidebarTrigger import (NAV-04)
- Replace plain h2 with gradient span wordmark using oklch 260-300 purple sweep (NAV-02)
- Override SidebarMenuButton active state with sidebar-primary for high contrast (NAV-03)
- Add header bar with SidebarTrigger inside SidebarInset (NAV-04)
- Add p-4 padding to main content area
- NAV-01 satisfied by existing bg-sidebar token on sidebar element

frontend/src/components/AppLayout.tsx

@@ -13,6 +13,7 @@ import {
   SidebarMenuItem,
   SidebarProvider,
   SidebarInset,
+  SidebarTrigger,
 } from '@/components/ui/sidebar'
 import { Button } from '@/components/ui/button'
 import type { AuthContext } from '@/hooks/useAuth'
@@ -36,7 +37,18 @@ export function AppLayout({ auth, children }: Props) {
     <SidebarProvider>
       <Sidebar>
         <SidebarHeader className="border-b px-4 py-3">
-          <h2 className="text-lg font-semibold">{t('app.title')}</h2>
+          <span
+            data-testid="sidebar-wordmark"
+            className="text-base font-semibold tracking-wide"
+            style={{
+              background: `linear-gradient(to right, oklch(0.50 0.12 260), oklch(0.50 0.12 300))`,
+              WebkitBackgroundClip: 'text',
+              WebkitTextFillColor: 'transparent',
+              backgroundClip: 'text',
+            }}
+          >
+            {t('app.title')}
+          </span>
           {auth.user && (
             <p className="text-sm text-muted-foreground">{auth.user.display_name}</p>
           )}
@@ -47,7 +59,11 @@ export function AppLayout({ auth, children }: Props) {
               <SidebarMenu>
                 {navItems.map((item) => (
                   <SidebarMenuItem key={item.path}>
-                    <SidebarMenuButton asChild isActive={location.pathname === item.path}>
+                    <SidebarMenuButton
+                      asChild
+                      isActive={location.pathname === item.path}
+                      className="data-[active=true]:bg-sidebar-primary data-[active=true]:text-sidebar-primary-foreground"
+                    >
                       <Link to={item.path}>
                         <item.icon />
                         <span>{item.label}</span>
@@ -67,7 +83,10 @@ export function AppLayout({ auth, children }: Props) {
         </SidebarFooter>
       </Sidebar>
       <SidebarInset>
-        <main className="flex-1">{children}</main>
+        <header className="flex h-12 shrink-0 items-center gap-2 border-b px-4">
+          <SidebarTrigger className="-ml-1" />
+        </header>
+        <main className="flex-1 p-4">{children}</main>
       </SidebarInset>
     </SidebarProvider>
   )