feat(settings): modernize download location settings

- Replace split 'Default Download Path' and 'All Categories Base' with a canonical baseDownloadFolder and categorySubfolders model
- Retain absolute category overrides only where required
- Update Add window to accurately reflect intended destination:
  - Single URL: specific category path
  - Multiple URLs: base folder with explanatory text
  - Manual Browse override: selected folder
- Centralize path resolution logic shared by Rust backend and UI

src/ipc.ts

@@ -101,7 +101,10 @@ type CommandMap = {
   db_replace_downloads: { args: { data: string }; result: void };
   db_get_all_queues: { args: undefined; result: string[] };
   db_replace_queues: { args: { data: string }; result: void };
-  create_category_directories: { args: { paths: string[] }; result: void };
+  create_category_directories: {
+    args: { baseFolder: string; subfolders: Record<string, string> };
+    result: void;
+  };
   export_logs: { args: { destPath: string }; result: string };
   get_pending_order: { args: undefined; result: string[] };
   enqueue_download: { args: { item: any }; result: string };