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/client/core/ApiError.ts

@@ -0,0 +1,21 @@
+import type { ApiRequestOptions } from './ApiRequestOptions';
+import type { ApiResult } from './ApiResult';
+
+export class ApiError extends Error {
+	public readonly url: string;
+	public readonly status: number;
+	public readonly statusText: string;
+	public readonly body: unknown;
+	public readonly request: ApiRequestOptions;
+
+	constructor(request: ApiRequestOptions, response: ApiResult, message: string) {
+		super(message);
+
+		this.name = 'ApiError';
+		this.url = response.url;
+		this.status = response.status;
+		this.statusText = response.statusText;
+		this.body = response.body;
+		this.request = request;
+	}
+}