feat: add category management page (create/edit/delete)

The backend already had full category CRUD but nothing in the UI exposed
it — categories could only be viewed inside charts. Adds a Categories nav
item and page with icon/color pickers, grouped by expense/income, backed
by new create/update/delete mutations. Also expands icons.tsx's ICON_MAP,
which was missing 14 icon keys already in use by real category data and
silently falling back to a wallet glyph everywhere they render.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "aurum-frontend",
   "private": true,
-  "version": "0.22.0",
+  "version": "0.23.0",
   "type": "module",
   "scripts": {
     "dev": "vite",