Add a barHeight setting to cap bar travel

[epicbagel]

Problem

track is pinned to the widget's full height minus 8px, with no way to tune it:

readonly property int track: Math.max(4, (vertical ? width : height) - 8)

Bars grow upward from the bottom of that track. On a bar taller than the 26px
default, that sinks the visualizer's mass to the bottom edge while every
neighbouring icon stays vertically centred, so Cava reads as misaligned with the
rest of the bar.

I hit this on a 70px bar (a 5K display at scale 1.6, plus a floating bar inset).

Measurements

Measured from screenshots by scanning pixel columns, same audio source, bar slab
centre at y=54:

	barHeight: 0 (today)	barHeight: 20
Baseline	y=79	y=69
Visible mass centre	y=66.5	y=56.5
Offset from bar centre	12.5px low	2.5px

Change

Adds a barHeight setting that caps how far a bar can travel:

readonly property int track: {
  var available = Math.max(4, (vertical ? width : height) - 8)
  return barHeight > 0 ? Math.min(barHeight, available) : available
}

• Defaults to 0, which keeps the current fill-the-bar behaviour exactly. No
  existing config changes appearance.
• Clamped with Math.min(barHeight, available), so it can never overflow the bar.
• Also clamped to 0..128 via the existing clampInt helper.
• Exposed in the popup menu next to Bar width, and declared in the manifest
  defaults and schema so it shows up in plugin settings UI.
• README settings table and a short note on why you'd reach for it.

Applies to vertical bars too, where it caps bar length rather than height.

Testing

• omarchy plugin validate passes.
• Verified live with audio playing: baseline moved to the predicted position
  (centre + track/2 = 70, measured 69), and bar length stayed within the new
  budget.
• Confirmed barHeight: 0 renders identically to before the change.

One note for anyone testing: the widget kept the old track after
omarchy-shell shell rescanPlugins; it only picked up the QML change after
omarchy restart shell.