Apply Petrol/Volt/Kupfer design system and project sidebar

Bring the Econsult-family design tokens (see design-system-econsult.md)
into leistungsbilanz-ts and add a persistent sidebar for the projects
area, mirroring elt-planung-suite / Einsatz-Orga. The sidebar lists all
projects and, inside a project, jumps to its sections; the tree editor
stays sidebar-free since it needs the full width. The projects overview
page is decluttered to a single stacked column (create/import/global
devices) now that the project list lives in the sidebar.

src/app/layout.tsx

@@ -1,6 +1,7 @@
 import type { Metadata } from "next";
 import "bootstrap/dist/css/bootstrap.min.css";
 import "./globals.css";
+import { AppShell } from "../frontend/components/AppShell";
 
 export const metadata: Metadata = {
   title: "Leistungsbilanz",
@@ -10,7 +11,9 @@ export const metadata: Metadata = {
 export default function RootLayout({ children }: Readonly<{ children: React.ReactNode }>) {
   return (
     <html lang="de">
-      <body>{children}</body>
+      <body>
+        <AppShell>{children}</AppShell>
+      </body>
     </html>
   );
 }