feat(MilkdownEditor): add file upload support for documents and text files

Added new upload functionality to the editor supporting doc/docx/ppt/pptx/pdf/zip/txt/json files. Includes:
- New upload button with file input
- File type detection utilities (isTextFile, isConvertibleFile)
- Initial markdown sync with trailing whitespace normalization
- Warning messages for unsupported file types

src/utils/convert.js

@@ -0,0 +1,46 @@
+import { CONVERT_URL } from './config.js'
+
+const API_KEY = 'your-secret-key-here'
+
+function readFileAsBase64(file) {
+    return new Promise((resolve, reject) => {
+        const reader = new FileReader()
+        reader.onload = () => {
+            const dataUrl = typeof reader.result === 'string' ? reader.result : ''
+            const splitIndex = dataUrl.indexOf(',')
+            if (splitIndex === -1) {
+                reject(new Error('Invalid file data'))
+                return
+            }
+            resolve(dataUrl.slice(splitIndex + 1))
+        }
+        reader.onerror = () => reject(reader.error || new Error('Failed to read file'))
+        reader.readAsDataURL(file)
+    })
+}
+
+export async function convertFileToMarkdown(file) {
+    const base64 = await readFileAsBase64(file)
+    const res = await fetch(CONVERT_URL, {
+        method: 'POST',
+        headers: {
+            'Content-Type': 'application/json',
+            'X-API-Key': API_KEY,
+        },
+        body: JSON.stringify({
+            file: base64,
+            filename: file.name || 'document',
+        }),
+    })
+
+    if (!res.ok) {
+        const errorText = await res.text()
+        throw new Error(`HTTP ${res.status}: ${errorText}`)
+    }
+
+    const data = await res.json()
+    if (!data || typeof data.markdown !== 'string') {
+        throw new Error('No markdown returned')
+    }
+    return data.markdown
+}