Phase 4 red: ApiClient tests and stub

19 tests covering ApiClient's full public surface: default and custom
origins, X-Client-Package and X-Auth-Token headers, getJSON with query
params, postJSON with JSON body, ApiError on 4xx/5xx, streaming file
and thumbnail downloads, and self-hosted origin routing.

Tests inject a recording fetch via the constructor, so nothing hits the
network. The test file is documented to serve as canonical usage
reference per the development workflow.

src/api/client.ts

@@ -0,0 +1,57 @@
+// Stub: see the README "Development workflow" section for TDD policy.
+
+export interface ApiClientOptions {
+    apiOrigin?: string;
+    filesOrigin?: string;
+    thumbsOrigin?: string;
+    authToken?: string;
+    fetch?: typeof globalThis.fetch;
+    userAgent?: string;
+}
+
+export class ApiError extends Error {
+    readonly status: number;
+    readonly code?: string;
+    readonly requestID?: string;
+    readonly body?: unknown;
+    constructor(
+        _message: string,
+        _status: number,
+        _opts?: { code?: string; requestID?: string; body?: unknown },
+    ) {
+        super(_message);
+        this.status = _status;
+        this.code = _opts?.code;
+        this.requestID = _opts?.requestID;
+        this.body = _opts?.body;
+    }
+}
+
+export class ApiClient {
+    constructor(_opts?: ApiClientOptions) {
+        throw new Error("ApiClient not implemented");
+    }
+    setAuthToken(_token: string): void {
+        throw new Error("not implemented");
+    }
+    clearAuthToken(): void {
+        throw new Error("not implemented");
+    }
+    async getJSON<T>(
+        _path: string,
+        _query?: Record<string, string | number | undefined>,
+    ): Promise<T> {
+        throw new Error("not implemented");
+    }
+    async postJSON<T>(_path: string, _body: unknown): Promise<T> {
+        throw new Error("not implemented");
+    }
+    async getFileStream(_fileID: number): Promise<ReadableStream<Uint8Array>> {
+        throw new Error("not implemented");
+    }
+    async getThumbnailStream(
+        _fileID: number,
+    ): Promise<ReadableStream<Uint8Array>> {
+        throw new Error("not implemented");
+    }
+}