fix(log): stop the dev log reopening after it is closed

[sidlatau]

Closes #467

• The dev log window opens automatically only the first time; after that, output goes to a hidden buffer that :FlutterLogToggle shows on demand.
• Previously, if the buffer had been unloaded or deleted (:bd, :bw, :q with nohidden), the next output line, such as a hot reload, opened a new window.