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/vite.config.ts

@@ -0,0 +1,22 @@
+import path from "node:path"
+import tailwindcss from "@tailwindcss/vite"
+import { tanstackRouter } from "@tanstack/router-plugin/vite"
+import react from "@vitejs/plugin-react-swc"
+import { defineConfig } from "vite"
+
+// https://vitejs.dev/config/
+export default defineConfig({
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+  plugins: [
+    tanstackRouter({
+      target: "react",
+      autoCodeSplitting: true,
+    }),
+    react(),
+    tailwindcss(),
+  ],
+})