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/api/categories.ts

@@ -1,6 +1,18 @@
 import { api } from "@/api/client";
-import type { Category } from "@/types";
+import type { Category, CategoryInput, CategoryUpdateInput } from "@/types";
 
 export function fetchCategories() {
   return api.get<Category[]>("/categories");
 }
+
+export function createCategory(input: CategoryInput) {
+  return api.post<Category>("/categories", input);
+}
+
+export function updateCategory(id: number, input: CategoryUpdateInput) {
+  return api.patch<Category>(`/categories/${id}`, input);
+}
+
+export function deleteCategory(id: number) {
+  return api.delete<void>(`/categories/${id}`);
+}