[Studio] feat: add centralized ThemeContext and useTheme hook for dark mode management

[zhaohai666]

Summary

Introduce centralized theme management built on React Context, refactor scattered local dark mode useState inside MainLayout. The exported useTheme hook exposes darkMode, setDarkMode and toggleTheme, following the same design pattern as existing useLang i18n hook.

Changes

File	Type	Description
web/src/theme/ThemeContext.tsx	Added	Implement ThemeProvider and useTheme hook with TypeScript. Load initial theme preference from localStorage, fall back to system prefers-color-scheme. Automatically persist updated selection to localStorage.
web/src/layouts/MainLayout.tsx	Modified	Remove local useState(false) dark mode state, consume shared state via useTheme(). Delete redundant inline ConfigProvider theme wrapper (moved globally). Replace manual () => setDarkMode(!darkMode) calls with built-in toggleTheme.
web/src/main.tsx	Modified	Wrap root application with <ThemeProvider>. Lift Ant Design ConfigProvider theme logic to entry level; darkAlgorithm / defaultAlgorithm switching and dark-mode token overrides apply globally for the whole app.

Design Decisions

1. React Context instead of Redux
   The codebase already adopts React Context for i18n (LangContext) and Zustand for authentication & cluster state. Theme state is presentation-layer UI state and mainly consumed inside layout components. React Context keeps consistent architecture with internationalization module, avoiding heavy state library dependency for a single boolean flag.

2. localStorage persistence
   User selected theme will survive page refresh. Storage key: rocketmq-studio-theme.

3. System color scheme fallback
   For first-time visitors without saved preference, automatically detect OS appearance via window.matchMedia('(prefers-color-scheme: dark)').

4. Global root ConfigProvider
   Moving theme algorithm switching out of MainLayout guarantees all Ant Design components (including standalone modals created by ReactDOM.createRoot outside layout tree) correctly render dark style.

Hook API Usage

import { useTheme } from '../theme/ThemeContext';
const { darkMode, setDarkMode, toggleTheme } = useTheme();