V124 QUICK FF DELAY-0 EPICENTER PREVIEW

Base
- Built from v123.

What changed
- Quick finite-fault placement preview no longer treats C3 as the fixed epicenter.
- The mouse cursor now represents the active cell/cells whose delay is 0 seconds.
- If multiple active cells have delay 0, their centroid is used as the preview/application origin.
- If no delay-0 active cell exists, the code falls back to C3, then to the first active cell.
- Final application uses the same delay-0 origin as the preview, including depth calculation relative to that origin.
- CSV save comment now says the epicenter cell is the active delay-0 cell.
