refactor: remove FFmpeg dependencies and related video processing logic

- Deleted FFmpeg related packages from package.json and package-lock.json.
- Removed video transcoding and editing functionalities from FileContent.vue.
- Simplified video handling logic and error management in FileContent.vue.
- Added a clear button in MilkdownEditor.vue for clearing the editor content.
- Enhanced UniverPreview.vue to clear detached popups and mount nodes on destroy.
- Updated docBlockPlugin.ts to improve context handling for document blocks.
- Cleaned up vite.config.js by removing cross-origin isolation headers.

vite.config.js

@@ -1,17 +1,11 @@
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
 
-const crossOriginIsolationHeaders = {
-  'Cross-Origin-Embedder-Policy': 'require-corp',
-  'Cross-Origin-Opener-Policy': 'same-origin'
-}
-
 export default defineConfig({
   plugins: [vue()],
   server: {
     host: true,
     port: 5173,
-    headers: crossOriginIsolationHeaders,
     proxy: {
       '/v1': {
         target: 'https://api.imageteach.tech:8002',
@@ -19,9 +13,6 @@ export default defineConfig({
       }
     }
   },
-  preview: {
-    headers: crossOriginIsolationHeaders
-  },
   build: {
     rollupOptions: {
       output: {