Add category management UI and service layer

Users can now create, edit, and delete custom categories from Settings → Categories. System categories (45 built-in) are shown read-only. Backend adds update_category() and delete_category() service functions; frontend has a new categories API module and a full CRUD section in SettingsPage with filter tabs, colour picker, and delete confirmation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -23,6 +23,7 @@ Runs entirely on your own hardware via Docker Compose. Designed for LAN access w
 - Per-category budget tracking with rollover support
 - Visual radial gauges showing spend vs. budget
 - Configurable alert threshold (default 80%)
+- Edit existing budgets (amount, period, alert threshold) without losing history
 
 ### Investments
 - Portfolio tracking with holdings and buy/sell/dividend/split transactions
@@ -31,10 +32,17 @@ Runs entirely on your own hardware via Docker Compose. Designed for LAN access w
 - Hourly FX rates for multi-currency conversion to GBP base
 - TWRR and MWRR performance metrics
 - Capital gains reporting (short/long-term by tax year)
-- OHLCV candlestick charts per asset
+- OHLCV candlestick charts per asset with buy/sell transaction history overlay
 - Portfolio visualisations: allocation donut by holding, allocation donut by asset type, cost basis vs current value bar chart, return % per holding bar chart
 - Investment account balances include holding market values in net worth, balance sheet, and accounts list — supports both simple (holdings only) and full cash-flow tracking workflows
 
+### Reports
+### Categories
+- 45 built-in system categories across income, expense, and transfer types
+- Create custom categories with a name, type, and colour
+- Rename and recolour existing custom categories
+- Managed in **Settings → Categories**
+
 ### Reports
 Seven report views:
 1. Net Worth over time (area chart with time slider)