[Enhancement]: Reorganise the transport footer into labelled control clusters #370

Problem

The transport footer put its playback controls in one wide row and hid the
click-track options behind a right-click popover. Two consequences:

• Nothing told you the click-track options existed. A right-click on a small
  pill is not an affordance anyone finds by accident.
• The controls competed for width in a single line, so the track title, the
  time readout, the speed control and the export button all shrank together
  on a narrower window.

Playback speed was also a continuous dial, which is the wrong shape for the
job — practising a part means slowing to a known fraction, not sweeping
tempo.

What it should be

Controls grouped into labelled clusters, everything visible in the bar:

• Transport — stop, play.
• Position — elapsed / total, plus the loop controls that act on that
  position (loop is a property of a position, not a transport mode).
• Speed — three practice presets (0.25x / 0.5x / 1x) as a segmented
  control, not a dial.
• Click track — on/off, count-in, accent, grid, rate and volume, all
  inline once a track has a beat grid. No popover.

On/off state should read the same way everywhere: click track, count-in and
grid are all press-to-toggle buttons with one shape and one lit state, rather
than a mix of buttons, sliding switches and one-way actions.

Notes

Addressed in #369. Filed after the fact so the work is tracked — the layout
came out of a design exploration ("two ways to group the same controls"),
and the chosen option is the two-tier / labelled-cluster one.