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.

frontend/src/hooks/useCustomToast.ts

@@ -0,0 +1,19 @@
+import { toast } from "sonner"
+
+const useCustomToast = () => {
+  const showSuccessToast = (description: string) => {
+    toast.success("Success!", {
+      description,
+    })
+  }
+
+  const showErrorToast = (description: string) => {
+    toast.error("Something went wrong!", {
+      description,
+    })
+  }
+
+  return { showSuccessToast, showErrorToast }
+}
+
+export default useCustomToast