feat(ui-flow): attachment download IPC for save-to-disk

Handle request-attachment-download using readAttachmentBytesWithSource
for any mime type and reply with base64 payload or error.

Co-authored-by: Cursor <[email protected]>

index.js

@@ -1392,6 +1392,29 @@ async function dispatchUiMessage (platform, msg, hooks = {}) {
     })
     return true
   }
+  if (t === 'request-attachment-download') {
+    const attachmentId = msg.attachmentId
+    try {
+      const row = await platform.attachments?.get(attachmentId)
+      if (!row) throw new Error('attachment not found')
+      const { buf, source } = await platform.readAttachmentBytesWithSource(attachmentId)
+      send({
+        type: 'attachment-download',
+        attachmentId: row.id,
+        filename: row.filename || 'download',
+        mimeType: row.mimeType || 'application/octet-stream',
+        dataBase64: b4a.toString(buf, 'base64'),
+        fetchSource: source
+      })
+    } catch (err) {
+      send({
+        type: 'attachment-download-error',
+        attachmentId,
+        message: err?.message || String(err)
+      })
+    }
+    return true
+  }
   if (t === 'upsert-guild-emoji') {
     await platform.upsertGuildEmoji({
       name: msg.name,