754: Color Assignment Files for Nodes

[glstott]

This addresses the feature request #754, adding a way for users to use a color assignment file for node coloring. The accepted formats are CSV, TSV, and iTOL files. In addition, this adds a MLST option to the distance dropdown, mapped to SNP distance calculation. This is to help users who are not familiar with SNP distance. Upon selection, a little info box appears to inform the user what is happening under the hood.

Summary

• Added reusable node color-assignment file imports under Global Settings → Styling.
• Supports:
  • iTOL DATASET_COLORSTRIP files with space, tab, or comma separators.
  • CSV/TSV tables using the first column as the matching value and a required color column.
• Added atomic validation for malformed rows, invalid hex colors, unsupported delimiters, and conflicting duplicate assignments.
• Stores assignments per node field, merges partial imports, retains mappings for future data, and persists them in saved sessions.
• Immediately refreshes rendered node colors and color tables after import while preserving unmapped colors.
• Prevents imported colors from leaking between fields with identical values.
• Added success/error import feedback with matched, unmapped, and retained assignment counts.
• Added unit coverage and Cypress fixtures/journeys for parsing, mapping precedence, session persistence, ID matching, partial imports, and invalid files.
• Added MLST to the File Settings and Global Settings distance dropdowns.
• Routes MLST through the existing SNP distance calculation, including SNP thresholds and step sizes.
• Displays an unobtrusive MLST/SNP equivalence note only while MLST is selected.
• Updated the documentation map with color-assignment example entries.