Handle a command task's end-of-input on the UI thread

When a command task reaches EOF it runs onEndOfInput, which reads the
view's line height (and thus its dimensions) to decide whether to scroll,
sets the view's origin, and flushes stale cells. Reading the dimensions
and setting the origin are UI-thread-only, but this ran on the task's own
goroutine, racing the UI thread. Bounce onEndOfInput onto the UI thread,
as we already do for the new-task origin reset. It's once per render, so
it doesn't add the per-line UI-thread churn that streaming the content
would.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/tasks/tasks.go

@@ -353,8 +353,14 @@ func (self *ViewBufferManager) NewCmdTask(start func() (Cmd, io.Reader), prefix
 
 						if !ok {
 							// if we're here then there's nothing left to scan from the source
-							// so we're at the EOF and can flush the stale content
-							self.onEndOfInput()
+							// so we're at the EOF and can flush the stale content.
+							// onEndOfInput reads the view's dimensions (to decide
+							// whether to scroll) and sets the origin, both of which
+							// are UI-thread-only, so run it there.
+							_ = self.onUIThread(func() error {
+								self.onEndOfInput()
+								return nil
+							})
 							callThen()
 							break outer
 						}