Refactor default headers to be injected dynamically (#367)

src-web/lib/defaultHeaders.ts

@@ -0,0 +1,8 @@
+import type { HttpRequestHeader } from '@yaakapp-internal/models';
+import { invokeCmd } from './tauri';
+
+/**
+ * Global default headers fetched from the backend.
+ * These are static and fetched once on module load.
+ */
+export const defaultHeaders: HttpRequestHeader[] = await invokeCmd('cmd_default_headers');