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/routes/_layout.tsx

@@ -0,0 +1,42 @@
+import { createFileRoute, Outlet, redirect } from "@tanstack/react-router"
+
+import { Footer } from "@/components/Common/Footer"
+import AppSidebar from "@/components/Sidebar/AppSidebar"
+import {
+  SidebarInset,
+  SidebarProvider,
+  SidebarTrigger,
+} from "@/components/ui/sidebar"
+import { isLoggedIn } from "@/hooks/useAuth"
+
+export const Route = createFileRoute("/_layout")({
+  component: Layout,
+  beforeLoad: async () => {
+    if (!isLoggedIn()) {
+      throw redirect({
+        to: "/login",
+      })
+    }
+  },
+})
+
+function Layout() {
+  return (
+    <SidebarProvider>
+      <AppSidebar />
+      <SidebarInset>
+        <header className="sticky top-0 z-10 flex h-16 shrink-0 items-center gap-2 border-b px-4">
+          <SidebarTrigger className="-ml-1 text-muted-foreground" />
+        </header>
+        <main className="flex-1 p-6 md:p-8">
+          <div className="mx-auto max-w-7xl">
+            <Outlet />
+          </div>
+        </main>
+        <Footer />
+      </SidebarInset>
+    </SidebarProvider>
+  )
+}
+
+export default Layout