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/hooks/useCategories.ts

@@ -1,6 +1,46 @@
-import { useQuery } from "@tanstack/react-query";
-import { fetchCategories } from "@/api/categories";
+import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { createCategory, deleteCategory, fetchCategories, updateCategory } from "@/api/categories";
+import type { CategoryInput, CategoryUpdateInput } from "@/types";
 
 export function useCategories() {
   return useQuery({ queryKey: ["categories"], queryFn: fetchCategories });
 }
+
+// Category name/color/icon are denormalized into several read models
+// (transactions, dashboard, reports, budgets, recurring templates) — any
+// create/update/delete has to refresh all of them, same set an account
+// mutation invalidates in useAccounts.ts.
+function invalidateCategoryConsumers(queryClient: ReturnType<typeof useQueryClient>) {
+  queryClient.invalidateQueries({ queryKey: ["categories"] });
+  queryClient.invalidateQueries({ queryKey: ["transactions"] });
+  queryClient.invalidateQueries({ queryKey: ["dashboard-summary"] });
+  queryClient.invalidateQueries({ queryKey: ["category-spending-report"] });
+  queryClient.invalidateQueries({ queryKey: ["category-ranking"] });
+  queryClient.invalidateQueries({ queryKey: ["budgets"] });
+  queryClient.invalidateQueries({ queryKey: ["budget-status"] });
+  queryClient.invalidateQueries({ queryKey: ["recurring"] });
+}
+
+export function useCreateCategory() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (input: CategoryInput) => createCategory(input),
+    onSuccess: () => invalidateCategoryConsumers(queryClient),
+  });
+}
+
+export function useUpdateCategory() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: ({ id, input }: { id: number; input: CategoryUpdateInput }) => updateCategory(id, input),
+    onSuccess: () => invalidateCategoryConsumers(queryClient),
+  });
+}
+
+export function useDeleteCategory() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (id: number) => deleteCategory(id),
+    onSuccess: () => invalidateCategoryConsumers(queryClient),
+  });
+}