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/js/api.js

@@ -25,6 +25,7 @@ export const api = {
   categories: (all = false) => req('GET', `/api/categories${all ? '?all=1' : ''}`),
   addCategory: (c) => req('POST', '/api/categories', c),
   updateCategory: (id, c) => req('PUT', `/api/categories/${id}`, c),
+  deleteCategory: (id) => req('DELETE', `/api/categories/${id}`),
   addMetric: (catId, m) => req('POST', `/api/categories/${catId}/metrics`, m),
   updateMetric: (id, m) => req('PUT', `/api/metrics/${id}`, m),
   deleteMetric: (id) => req('DELETE', `/api/metrics/${id}`),