v0.14.5 — preview header fixes

Three display bugs, all found while rendering the README screenshot. All three showed on every connected-clip project and none were visible against examples/sample.fcpxml.

• The header read "0 clips" on a timeline holding 129 of them. Timeline.total_clips counts spine clips only, so every music-video-shaped project reported zero. It now reports the real total and how many lanes they span.
• An empty spine rendered as a grey box taking a third of the frame, on projects where nothing sits on the spine by design. The row is omitted when there is nothing to draw.
• Frame rate printed as 23.976023976023978fps.

Fourth time in one day that fixture hid something, which is why v0.14.4 added one shaped like real work.

Full Changelog: v0.14.4...v0.14.5