Add Start Marker Editing in Audio Clip View

[m0nkmaster]

Adds the ability to edit the starting point of audio clips directly from the clip view, similar to how end points are currently edited. This makes manipulating audio clip trimming more intuitive and efficient and removes the need for the "reverse the clip then crop the end, revert reverse" workaround. See: https://www.youtube.com/watch?v=JoLCEb-pv-A&t=1304s. Here's another workaround using Arranger view: https://www.youtube.com/watch?v=iWhVUsx40Mg&t=76s .

Changes

• Added startMarkerVisible state to track when start marker is being edited
• Implemented start marker showing in blue (end marker stays red)
• Modified waveform rendering to respect both start and end markers
• Added proper cleanup of clip playback state during marker edits

Usage

To edit the start point:

1. Press any pad in the first column where your audio clip starts
2. The start marker will appear in blue
3. Press another pad to move the start point
4. Press the same pad again to hide the marker

To edit the end point (unchanged):

1. Press any pad in the last column where your audio clip ends
2. The end marker will appear in red
3. Press another pad to move the end point
4. Press the same pad again to hide the marker