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/Common/ErrorComponent.tsx

@@ -0,0 +1,29 @@
+import { Link } from "@tanstack/react-router"
+import { Button } from "@/components/ui/button"
+
+const ErrorComponent = () => {
+  return (
+    <div
+      className="flex min-h-screen items-center justify-center flex-col p-4"
+      data-testid="error-component"
+    >
+      <div className="flex items-center z-10">
+        <div className="flex flex-col ml-4 items-center justify-center p-4">
+          <span className="text-6xl md:text-8xl font-bold leading-none mb-4">
+            Error
+          </span>
+          <span className="text-2xl font-bold mb-2">Oops!</span>
+        </div>
+      </div>
+
+      <p className="text-lg text-muted-foreground mb-4 text-center z-10">
+        Something went wrong. Please try again.
+      </p>
+      <Link to="/">
+        <Button>Go Home</Button>
+      </Link>
+    </div>
+  )
+}
+
+export default ErrorComponent