Vendor backend and frontend into the monorepo

The submodule collapse was only half applied: .gitmodules was deleted but
backend/ and frontend/ were still recorded as gitlinks, so none of their
files were tracked. Replace the gitlinks with the real trees.

Also untrack .env (it carried placeholder secrets) in favour of a tracked
.env.example, drop the committed __pycache__, and narrow the blanket *.png
ignore that would have swallowed design assets.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/components/Sidebar/AppSidebar.tsx

@@ -0,0 +1,43 @@
+import { Briefcase, Home, Users } from "lucide-react"
+
+import { SidebarAppearance } from "@/components/Common/Appearance"
+import { Logo } from "@/components/Common/Logo"
+import {
+  Sidebar,
+  SidebarContent,
+  SidebarFooter,
+  SidebarHeader,
+} from "@/components/ui/sidebar"
+import useAuth from "@/hooks/useAuth"
+import { type Item, Main } from "./Main"
+import { User } from "./User"
+
+const baseItems: Item[] = [
+  { icon: Home, title: "Dashboard", path: "/" },
+  { icon: Briefcase, title: "Items", path: "/items" },
+]
+
+export function AppSidebar() {
+  const { user: currentUser } = useAuth()
+
+  const items = currentUser?.is_superuser
+    ? [...baseItems, { icon: Users, title: "Admin", path: "/admin" }]
+    : baseItems
+
+  return (
+    <Sidebar collapsible="icon">
+      <SidebarHeader className="px-4 py-6 group-data-[collapsible=icon]:px-0 group-data-[collapsible=icon]:items-center">
+        <Logo variant="responsive" />
+      </SidebarHeader>
+      <SidebarContent>
+        <Main items={items} />
+      </SidebarContent>
+      <SidebarFooter>
+        <SidebarAppearance />
+        <User user={currentUser} />
+      </SidebarFooter>
+    </Sidebar>
+  )
+}
+
+export default AppSidebar