feat: optimize GGA overhangs and list all part versions in assembly

- Optimize linker-junction overhang selection: reject palindromes and
  rank candidates by Hamming distance to already-committed overhangs so
  adjacent overhangs in an assembly are maximally distinct (less
  cross-reactivity), with warnings when separation is insufficient.
- List every version of every part as its own insert (code + GC% + copy)
  in the GGA assembly, defaulting the FASTA export selection to just the
  active version of each part; per-insert checkboxes and All/Active-only/
  None quick actions govern export.
- Add a "None" option to the prefix/suffix standard dropdown.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: cborac