main: 增加全局暗黑模式支持及 Markdown 渲染功能

- 实现暗黑模式切换逻辑，支持系统主题适配
- 优化全局样式，添加动态主题变量及暗黑样式
- 引入 `markdown-it` 和 `mermaid`，支持 Markdown 内容及流程图渲染
- 更新 ChatView 测试用例，验证 Markdown 和 Mermaid 渲染效果
- 修改 package.json，增加新依赖

tests/chat-view.spec.js

@@ -12,6 +12,12 @@ vi.mock('axios', () => ({
     post: vi.fn(),
   },
 }))
+vi.mock('mermaid', () => ({
+  default: {
+    initialize: vi.fn(),
+    run: vi.fn().mockResolvedValue(),
+  },
+}))
 
 const mockedAxios = axios
 let latestEventSource = null
@@ -451,6 +457,67 @@ describe('ChatView message.delta handling', () => {
   })
 })
 
+describe('ChatView markdown rendering', () => {
+  beforeEach(() => {
+    localStorage.clear()
+    vi.clearAllMocks()
+    latestEventSource = null
+    globalThis.EventSource = MockEventSource
+    globalThis.scrollTo = vi.fn()
+  })
+
+  it('renders markdown content and mermaid blocks', async () => {
+    localStorage.setItem('ars-token', 'token')
+
+    const router = makeRouter()
+    router.push('/chat/session-1')
+    await router.isReady()
+
+    const markdownText = ['Hello **world**', '', '```mermaid', 'graph TD;', 'A-->B;', '```'].join(
+      '\n'
+    )
+    const list = [
+      buildMessage({
+        seq: 1,
+        role: 'AGENT',
+        type: 'agent.message',
+        content: markdownText,
+      }),
+    ]
+
+    mockedAxios.get.mockImplementation((url) => {
+      if (url.endsWith('/sessions/session-1')) {
+        return Promise.resolve({
+          data: { session_name: 'Demo', status: 'OPEN', last_seq: 1 },
+        })
+      }
+      if (url.endsWith('/sessions/session-1/messages')) {
+        return Promise.resolve({ data: { data: list } })
+      }
+      return Promise.resolve({ data: {} })
+    })
+
+    const wrapper = mount(ChatView, {
+      global: {
+        plugins: [router, ElementPlus],
+      },
+    })
+
+    await flushPromises()
+    await nextTick()
+
+    const bubble = wrapper.find('.bubble-agent')
+    expect(bubble.exists()).toBe(true)
+
+    const textBlock = bubble.find('.bubble-text')
+    expect(textBlock.html()).toContain('<strong>world</strong>')
+
+    const mermaidBlock = textBlock.find('.mermaid')
+    expect(mermaidBlock.exists()).toBe(true)
+    expect(mermaidBlock.text()).toContain('A-->B')
+  })
+})
+
 describe('ChatView system error and failed status handling', () => {
   beforeEach(() => {
     localStorage.clear()