Refactor error handling and localDb structure, and fix usage tracking bug.

src/lib/dataDir.js

@@ -0,0 +1,14 @@
+import path from "path";
+import os from "os";
+
+const APP_NAME = "9router";
+
+export function getDataDir() {
+  if (process.env.DATA_DIR) return process.env.DATA_DIR;
+  if (process.platform === "win32") {
+    return path.join(process.env.APPDATA || path.join(os.homedir(), "AppData", "Roaming"), APP_NAME);
+  }
+  return path.join(os.homedir(), `.${APP_NAME}`);
+}
+
+export const DATA_DIR = getDataDir();