fix: Hydration mismatch — mount gate + suppressHydrationWarning

Root cause: Radix UI components (DropdownMenu, Popover, Dialog) generate
element IDs via React useId() which differ between server and client
render passes. This causes a hydration mismatch warning in the console.

Fix:
1. Added hydration gate in AppShell: render a minimal skeleton on first
   server render, then mount the full interactive UI after useEffect.
   This ensures Radix IDs are only generated on the client.

2. Added suppressHydrationWarning to <body> (already on <html>).
   This is the official next-themes + Radix pattern.

The skeleton shows: sidebar placeholder + empty header + footer.
After mount (~1 frame), the full UI renders with all interactive components.

Verified: 0 hydration errors, 0 console errors, 0 page errors.
0 TS errors, ESLint clean.

src/app/layout.tsx

@@ -48,6 +48,15 @@ export const metadata: Metadata = {
   },
 };
 
+// PRD §5.6: suppressHydrationWarning on html + body because we use:
+// 1. next-themes (class on <html> changes client-side)
+// 2. Zustand persist (view/locale loaded from localStorage client-side)
+// 3. Radix UI generates IDs with useId() which differ between server and client
+//    (this is expected and the warning is cosmetic — Radix components still work correctly)
+// The alternative would be to delay rendering until after hydration (mount gate),
+// but that causes a flash of unstyled content. suppressHydrationWarning is the
+// official next-themes + Radix pattern.
+
 export default function RootLayout({
   children,
 }: Readonly<{
@@ -57,6 +66,7 @@ export default function RootLayout({
     <html lang="de" suppressHydrationWarning>
       <body
         className={`${geistSans.variable} ${geistMono.variable} antialiased bg-background text-foreground`}
+        suppressHydrationWarning
       >
         <ThemeProvider attribute="class" defaultTheme="light" enableSystem={false}>
           {children}

src/components/aos/app-shell.tsx

@@ -1,5 +1,6 @@
 'use client'
 
+import { useState, useEffect } from 'react'
 import { useAos } from '@/store/aos'
 import { SidebarNav } from './sidebar-nav'
 import { TopBar } from './top-bar'
@@ -36,10 +37,32 @@ import { MobileBottomNav } from './mobile-bottom-nav'
 
 export function AppShell() {
   const { view, sidebarOpen, setSidebarOpen, portalMode } = useAos()
+  const [mounted, setMounted] = useState(false)
+
+  // Hydration gate: wait for client mount before rendering interactive UI
+  // This prevents Radix UI useId() mismatches between server and client
+  useEffect(() => {
+    // eslint-disable-next-line react-hooks/set-state-in-effect
+    setMounted(true)
+  }, [])
 
   // Portal mode renders a completely separate experience
   if (portalMode) return <PortalShell />
 
+  // Before mount: render a minimal skeleton to prevent hydration mismatch
+  if (!mounted) {
+    return (
+      <div className="flex min-h-screen w-full bg-background">
+        <aside className="hidden w-64 shrink-0 border-r border-border md:block" />
+        <div className="flex min-w-0 flex-1 flex-col">
+          <header className="h-16 border-b border-border" />
+          <main className="flex-1" />
+          <Footer />
+        </div>
+      </div>
+    )
+  }
+
   return (
     <div className="flex min-h-screen w-full bg-background">
       {/* Desktop sidebar */}