feat(frontend): Responsive Smartphone-Oberfläche (Modul 15)

Mobile Bottom-Navigation und permanent erreichbare "+ Neu"-
Schnellaktion (Durchsicht/Varroa/Behandlung/Fütterung/Aufgabe/Ernte).
Header-Navigation überarbeitet, damit sie bei schmaler Breite intern
scrollt statt die Seite horizontal zu sprengen. Verifiziert bei
Smartphone- (390px) und Tablet-Breite (820px).

ApiCore-Frontend/src/app/(app)/layout.tsx

@@ -5,6 +5,8 @@ import Link from "next/link";
 import { useRouter } from "next/navigation";
 import { useAuth } from "@/lib/auth/auth-context";
 import { Button } from "@/components/ui/button";
+import { BottomNav } from "./bottom-nav";
+import { QuickAddMenu } from "./quick-add-menu";
 
 export default function AppLayout({ children }: { children: ReactNode }) {
   const router = useRouter();
@@ -22,9 +24,10 @@ export default function AppLayout({ children }: { children: ReactNode }) {
 
   return (
     <div className="flex min-h-screen flex-col">
-      <header className="flex items-center justify-between border-b px-4 py-3">
-        <nav className="flex items-center gap-4">
-          <span className="font-semibold">ApiCore</span>
+      <header className="flex items-center justify-between gap-3 border-b px-4 py-3">
+        <nav className="flex min-w-0 items-center gap-4 max-sm:gap-2">
+          <span className="shrink-0 font-semibold">ApiCore</span>
+          <div className="hidden min-w-0 items-center gap-4 overflow-x-auto whitespace-nowrap sm:flex">
           <Link className="text-sm text-muted-foreground hover:text-foreground" href="/dashboard">
             Dashboard
           </Link>
@@ -60,9 +63,12 @@ export default function AppLayout({ children }: { children: ReactNode }) {
               Benutzer
             </Link>
           ) : null}
+          </div>
         </nav>
-        <div className="flex items-center gap-3 text-sm">
-          <span className="text-muted-foreground">{user.email}</span>
+        <div className="flex shrink-0 items-center gap-3 text-sm">
+          <span className="hidden max-w-40 truncate text-muted-foreground md:inline">
+            {user.email}
+          </span>
           <Button
             variant="outline"
             size="sm"
@@ -74,7 +80,9 @@ export default function AppLayout({ children }: { children: ReactNode }) {
           </Button>
         </div>
       </header>
-      <div className="flex-1">{children}</div>
+      <div className="flex-1 pb-16 sm:pb-0">{children}</div>
+      <BottomNav />
+      <QuickAddMenu />
     </div>
   );
 }