[ingvarch]

What changes

A file that breaks while urga follows it now shows its error on the file screen every time. Before, the error was sent from a goroutine of its own, and when the break left no half line waiting, the lines could close first: the screen then showed a plain end.

• File sends the error from the goroutine that closes the lines, after the last line and before they close. That goroutine is followFrames in internal/nomad/files.go, which the agent log stream (followAgent) now shares instead of having its own copy.
• A refused file stream (no frames at all) also closes its lines after its error, as the agent log does.
• Tests: through File, a stream that breaks after its lines sends its error after the last line and before the end (the old code failed it on every run); followFrames with the frames closed before the error comes keeps the lines open until it does; a refused stream ends with its error. They pass with -count=50 -race.

Left as it is, as the issue says: task logs. Their error can still be taken before a last line that waits for its line break; that is a separate case.

Closes #176

Checklist

• The test for this change was written first and failed
• make check is green
• The title is a conventional commit: feat(ui): ..., fix(nomad): ...