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/ui/sonner.tsx

@@ -0,0 +1,40 @@
+"use client"
+
+import {
+  CircleCheckIcon,
+  InfoIcon,
+  Loader2Icon,
+  OctagonXIcon,
+  TriangleAlertIcon,
+} from "lucide-react"
+import { useTheme } from "next-themes"
+import { Toaster as Sonner, type ToasterProps } from "sonner"
+
+const Toaster = ({ ...props }: ToasterProps) => {
+  const { theme = "system" } = useTheme()
+
+  return (
+    <Sonner
+      theme={theme as ToasterProps["theme"]}
+      className="toaster group"
+      icons={{
+        success: <CircleCheckIcon className="size-4" />,
+        info: <InfoIcon className="size-4" />,
+        warning: <TriangleAlertIcon className="size-4" />,
+        error: <OctagonXIcon className="size-4" />,
+        loading: <Loader2Icon className="size-4 animate-spin" />,
+      }}
+      style={
+        {
+          "--normal-bg": "var(--popover)",
+          "--normal-text": "var(--popover-foreground)",
+          "--normal-border": "var(--border)",
+          "--border-radius": "var(--radius)",
+        } as React.CSSProperties
+      }
+      {...props}
+    />
+  )
+}
+
+export { Toaster }