Replace gene dropdown with segment dropdown in NT fitness heatmap dashboard (#57)

- Swap protein dropdown for segment dropdown (HA, NA, NP, PA, PB1, PB2, MP, NS)
- Subtype dropdown now depends on segment selection
- Filter by segment instead of gene; add Gene to heatmap tooltip
- Drop _GENE_TO_SEGMENT mapping; look up reference NT directly by segment:subtype
- Default mutation class changed to synonymous
- Regenerate WASM HTML

Closes #56

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Haddox