V123 QUICK FF MW PREVIEW DEFINE FIX

Fixes the v122 compile/link warning/error where quick_ff_update_mw_preview() was declared and called but not actually defined.

The restored helper computes a quick moment/magnitude preview from active quick finite-fault cells:
- M0 = rigidity * patch_length * patch_width * slip for each active cell
- Mw = 2/3 * (log10(M0) - 9.10)

No intended behavior change besides making the magnitude preview function real and compile-safe.
