feat(attachments): composer focus after stage-attachment (Phase 522)

Emit composer-focus-channel when stage-attachment completes in guild
mode so file uploads restore composer autofocus like Discord.

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

index.js

@@ -700,6 +700,12 @@ async function dispatchUiMessage (platform, msg, hooks = {}) {
       filename: msg.filename,
       mimeType: msg.mimeType
     })
+    if (platform.activeChannelId && platform.mode === 'guild') {
+      send({
+        type: 'composer-focus-channel',
+        channelId: platform.activeChannelId
+      })
+    }
     send({
       type: 'attachment-staged',
       attachment: att,