feat: add 4 default income categories, localize category display names

Fresh installs (this app is published on GitHub) only got Salary,
Freelance, Investments, Gifts, Other Income by default — adds Business
Income, Rental Income, Benefits, and Item Sales, rounding out the 8-slot
categorical palette. Only affects new databases (seeding is a one-time
no-op once any category exists).

Also fixes CategoryList.tsx (added last commit) to run category names
through translateCategoryName() like every other category display in the
app, and registers the new defaults' translation keys.

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

frontend/src/lib/i18n.ts

@@ -324,11 +324,10 @@ const ru = {
   "cashFlow.net": "Баланс",
   "cashFlow.noData": "Нет данных за выбранный период.",
 
-  // Default seeded categories (backend/app/db/seed.py) — there's no
-  // category-management UI yet, so every category currently in the
-  // database is one of these; translateCategoryName() matches on the exact
-  // seeded English name and falls through unchanged for anything else
-  // (a renamed default, or a future custom category).
+  // Default seeded categories (backend/app/db/seed.py) — translateCategoryName()
+  // matches on the exact seeded English name and falls through unchanged
+  // for anything else (a renamed default, or a custom category added via
+  // the category-management UI).
   "category.housingUtilities": "Жильё и коммуналка",
   "category.groceries": "Продукты",
   "category.diningOut": "Кафе и рестораны",
@@ -341,6 +340,10 @@ const ru = {
   "category.freelance": "Фриланс",
   "category.investments": "Инвестиции",
   "category.gifts": "Подарки",
+  "category.businessIncome": "Доход от бизнеса",
+  "category.rentalIncome": "Сдача недвижимости",
+  "category.benefits": "Пособия и соцвыплаты",
+  "category.itemSales": "Продажа личных вещей",
   "category.otherIncome": "Прочий доход",
 
   "months.jan": "Янв",
@@ -689,6 +692,10 @@ const en: Record<keyof typeof ru, string> = {
   "category.freelance": "Freelance",
   "category.investments": "Investments",
   "category.gifts": "Gifts",
+  "category.businessIncome": "Business Income",
+  "category.rentalIncome": "Rental Income",
+  "category.benefits": "Benefits",
+  "category.itemSales": "Item Sales",
   "category.otherIncome": "Other Income",
 
   "months.jan": "Jan",