refactor(dashboard): split chat.tsx into modular chat/ directory

- Extract types, utils, and components into separate files
- types.ts: All interfaces and type definitions (107 lines)
- utils.ts: Pure utility functions (50 lines)
- MessageRenderer.tsx: Message rendering components (96 lines)
- VirtualIdentityDialog.tsx: Virtual identity dialog (206 lines)
- ChatTabBar.tsx: Tab bar component (79 lines)
- index.tsx: Main ChatPage component (1147 lines)
- Update router import path to chat/index
- Fix TypeScript type imports per verbatimModuleSyntax
- Build passes with zero errors

dashboard/src/router.tsx

@@ -22,7 +22,7 @@ import { ModelPresetsPage } from './routes/model-presets'
 import { PluginConfigPage } from './routes/plugin-config'
 import { PluginMirrorsPage } from './routes/plugin-mirrors'
 import { PluginDetailPage } from './routes/plugin-detail'
-import { ChatPage } from './routes/chat'
+import { ChatPage } from './routes/chat/index'
 import { WebUIFeedbackSurveyPage, MaiBotFeedbackSurveyPage } from './routes/survey'
 import { AnnualReportPage } from './routes/annual-report'
 import PackMarketPage from './routes/config/pack-market'