feat: add JSON config module with load/save/defaults

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/data/config.test.ts

@@ -0,0 +1,55 @@
+import { describe, it, expect, afterEach } from "vitest";
+import { join } from "node:path";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { getDefaultConfig, loadConfig, saveConfig } from "./config.js";
+
+describe("config", () => {
+  const dirs: string[] = [];
+
+  function makeTmpDir(): string {
+    const dir = mkdtempSync(join(tmpdir(), "tsmusicbot-test-"));
+    dirs.push(dir);
+    return dir;
+  }
+
+  afterEach(() => {
+    for (const d of dirs) {
+      rmSync(d, { recursive: true, force: true });
+    }
+    dirs.length = 0;
+  });
+
+  it("returns default config when file does not exist", () => {
+    const config = loadConfig("/nonexistent/path/config.json");
+    expect(config).toEqual(getDefaultConfig());
+  });
+
+  it("creates config file on save", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "sub", "config.json");
+    const config = getDefaultConfig();
+    saveConfig(path, config);
+
+    const loaded = loadConfig(path);
+    expect(loaded).toEqual(config);
+  });
+
+  it("merges partial config with defaults", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+
+    // Save a partial config by writing only some fields
+    const partial = { webPort: 8080, locale: "en" };
+    const { writeFileSync } = require("node:fs");
+    writeFileSync(path, JSON.stringify(partial), "utf-8");
+
+    const loaded = loadConfig(path);
+    expect(loaded.webPort).toBe(8080);
+    expect(loaded.locale).toBe("en");
+    // defaults should fill in the rest
+    expect(loaded.theme).toBe("dark");
+    expect(loaded.commandPrefix).toBe("!");
+    expect(loaded.autoPauseOnEmpty).toBe(true);
+  });
+});