Add full category & metric management in Settings

- New Settings editor: rename categories (emoji/color too), rename
  metrics, change unit and type, add/remove metrics, set step + record
  tracking, archive/unarchive, and permanently delete categories.
- Settings now lists archived categories so they can be restored/deleted.
- Backend: add DELETE /api/categories/:id (cascades metrics, entries,
  entry_values, plans, goals).
- Bump StartOS package to 0.1.6:0; service worker cache to v6.

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'premier-gunner-v5';
+const CACHE = 'premier-gunner-v6';
 const SHELL = [
   '/', '/index.html', '/login.html',
   '/css/styles.css',