feat: add ZIP skill import with client-side extraction

Add import skill modal that accepts .zip files via drag-and-drop or
file picker, extracts them client-side using fflate, validates structure
and security constraints, then batch uploads via presigned URLs.

- Add fflate dependency for browser-side ZIP decompression
- Create zip-extract.ts with fflate filter API for validation
- Create zip-to-upload-tree.ts for BatchUploadNodeInput tree building
- Create import-skill-modal.tsx with drag-and-drop support
- Lazy-load ImportSkillModal via next/dynamic for bundle optimization
- Add en-US and zh-Hans i18n keys for import modal

web/app/components/workflow/skill/utils/zip-to-upload-tree.ts

@@ -0,0 +1,71 @@
+import type { ExtractedZipResult } from './zip-extract'
+import type { BatchUploadNodeInput } from '@/types/app-asset'
+import { getFileExtension } from './file-utils'
+import { prepareSkillUploadFile } from './skill-upload-utils'
+
+export type ZipUploadData = {
+  tree: BatchUploadNodeInput[]
+  files: Map<string, File>
+}
+
+function uint8ArrayToFile(data: Uint8Array, name: string): File {
+  const ext = getFileExtension(name)
+  const type = ext === 'md' || ext === 'markdown' || ext === 'mdx'
+    ? 'text/markdown'
+    : 'application/octet-stream'
+  const buffer = new ArrayBuffer(data.byteLength)
+  new Uint8Array(buffer).set(data)
+  return new File([buffer], name, { type })
+}
+
+export async function buildUploadDataFromZip(extracted: ExtractedZipResult): Promise<ZipUploadData> {
+  const fileMap = new Map<string, File>()
+  const tree: BatchUploadNodeInput[] = []
+  const folderMap = new Map<string, BatchUploadNodeInput>()
+
+  const entries = await Promise.all(
+    Array.from(extracted.files.entries()).map(async ([path, data]) => {
+      const fileName = path.split('/').pop()!
+      const rawFile = uint8ArrayToFile(data, fileName)
+      const prepared = await prepareSkillUploadFile(rawFile)
+      return { path, prepared }
+    }),
+  )
+
+  for (const { path, prepared } of entries) {
+    fileMap.set(path, prepared)
+
+    const parts = path.split('/')
+    let currentLevel = tree
+    let currentPath = ''
+
+    for (let i = 0; i < parts.length; i++) {
+      const part = parts[i]
+      const isLastPart = i === parts.length - 1
+      currentPath = currentPath ? `${currentPath}/${part}` : part
+
+      if (isLastPart) {
+        currentLevel.push({
+          name: part,
+          node_type: 'file',
+          size: prepared.size,
+        })
+      }
+      else {
+        let folder = folderMap.get(currentPath)
+        if (!folder) {
+          folder = {
+            name: part,
+            node_type: 'folder',
+            children: [],
+          }
+          folderMap.set(currentPath, folder)
+          currentLevel.push(folder)
+        }
+        currentLevel = folder.children!
+      }
+    }
+  }
+
+  return { tree, files: fileMap }
+}