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/tests/config.ts

@@ -0,0 +1,19 @@
+import path from "node:path"
+import { fileURLToPath } from "node:url"
+import dotenv from "dotenv"
+
+const __filename = fileURLToPath(import.meta.url)
+const __dirname = path.dirname(__filename)
+
+dotenv.config({ path: path.join(__dirname, "../../.env") })
+
+function getEnvVar(name: string): string {
+  const value = process.env[name]
+  if (!value) {
+    throw new Error(`Environment variable ${name} is undefined`)
+  }
+  return value
+}
+
+export const firstSuperuser = getEnvVar("FIRST_SUPERUSER")
+export const firstSuperuserPassword = getEnvVar("FIRST_SUPERUSER_PASSWORD")