move dropdown to navbar. show useful alerts.

Save vertical space by moving the Data Source dropdown up into the navbar.

Show an info alert when a file is loaded.

Show an info alert in live view with number of samples collected so far.

Position the alert so hiding / showing it doesn't move other page content.
data source dropdown: add some separators.
resize frequency band highlights after rendering, so that they always fit the canvas.
also, the frequency band buttons should stay toggled when loading a new file.