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/src/lib/navigation.ts

@@ -9,6 +9,7 @@ import {
   PieChart,
   Repeat,
   Settings,
+  Tags,
   Target,
   TrendingUp,
   type LucideIcon,
@@ -29,6 +30,7 @@ export const NAV_ITEMS: NavItem[] = [
   { labelKey: "nav.roi", to: "/roi", icon: Calculator },
   { labelKey: "nav.transactions", to: "/transactions", icon: ArrowLeftRight },
   { labelKey: "nav.accounts", to: "/accounts", icon: Layers },
+  { labelKey: "nav.categories", to: "/categories", icon: Tags },
   { labelKey: "nav.cashFlow", to: "/cash-flow", icon: Activity },
   { labelKey: "nav.reports", to: "/reports", icon: PieChart },
   { labelKey: "nav.budget", to: "/budget", icon: Target },