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/CancelablePromise.ts

@@ -0,0 +1,126 @@
+export class CancelError extends Error {
+	constructor(message: string) {
+		super(message);
+		this.name = 'CancelError';
+	}
+
+	public get isCancelled(): boolean {
+		return true;
+	}
+}
+
+export interface OnCancel {
+	readonly isResolved: boolean;
+	readonly isRejected: boolean;
+	readonly isCancelled: boolean;
+
+	(cancelHandler: () => void): void;
+}
+
+export class CancelablePromise<T> implements Promise<T> {
+	private _isResolved: boolean;
+	private _isRejected: boolean;
+	private _isCancelled: boolean;
+	readonly cancelHandlers: (() => void)[];
+	readonly promise: Promise<T>;
+	private _resolve?: (value: T | PromiseLike<T>) => void;
+	private _reject?: (reason?: unknown) => void;
+
+	constructor(
+		executor: (
+			resolve: (value: T | PromiseLike<T>) => void,
+			reject: (reason?: unknown) => void,
+			onCancel: OnCancel
+		) => void
+	) {
+		this._isResolved = false;
+		this._isRejected = false;
+		this._isCancelled = false;
+		this.cancelHandlers = [];
+		this.promise = new Promise<T>((resolve, reject) => {
+			this._resolve = resolve;
+			this._reject = reject;
+
+			const onResolve = (value: T | PromiseLike<T>): void => {
+				if (this._isResolved || this._isRejected || this._isCancelled) {
+					return;
+				}
+				this._isResolved = true;
+				if (this._resolve) this._resolve(value);
+			};
+
+			const onReject = (reason?: unknown): void => {
+				if (this._isResolved || this._isRejected || this._isCancelled) {
+					return;
+				}
+				this._isRejected = true;
+				if (this._reject) this._reject(reason);
+			};
+
+			const onCancel = (cancelHandler: () => void): void => {
+				if (this._isResolved || this._isRejected || this._isCancelled) {
+					return;
+				}
+				this.cancelHandlers.push(cancelHandler);
+			};
+
+			Object.defineProperty(onCancel, 'isResolved', {
+				get: (): boolean => this._isResolved,
+			});
+
+			Object.defineProperty(onCancel, 'isRejected', {
+				get: (): boolean => this._isRejected,
+			});
+
+			Object.defineProperty(onCancel, 'isCancelled', {
+				get: (): boolean => this._isCancelled,
+			});
+
+			return executor(onResolve, onReject, onCancel as OnCancel);
+		});
+	}
+
+	get [Symbol.toStringTag]() {
+		return "Cancellable Promise";
+	}
+
+	public then<TResult1 = T, TResult2 = never>(
+		onFulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | null,
+		onRejected?: ((reason: unknown) => TResult2 | PromiseLike<TResult2>) | null
+	): Promise<TResult1 | TResult2> {
+		return this.promise.then(onFulfilled, onRejected);
+	}
+
+	public catch<TResult = never>(
+		onRejected?: ((reason: unknown) => TResult | PromiseLike<TResult>) | null
+	): Promise<T | TResult> {
+		return this.promise.catch(onRejected);
+	}
+
+	public finally(onFinally?: (() => void) | null): Promise<T> {
+		return this.promise.finally(onFinally);
+	}
+
+	public cancel(): void {
+		if (this._isResolved || this._isRejected || this._isCancelled) {
+			return;
+		}
+		this._isCancelled = true;
+		if (this.cancelHandlers.length) {
+			try {
+				for (const cancelHandler of this.cancelHandlers) {
+					cancelHandler();
+				}
+			} catch (error) {
+				console.warn('Cancellation threw an error', error);
+				return;
+			}
+		}
+		this.cancelHandlers.length = 0;
+		if (this._reject) this._reject(new CancelError('Request aborted'));
+	}
+
+	public get isCancelled(): boolean {
+		return this._isCancelled;
+	}
+}