Use mapped HGNC name for assay facts gene text when available

[bencap]

This pull request improves how gene information is displayed in the AssayFactSheet.vue component. The main focus is to show a clearer and more accurate gene label, handling cases where multiple genes are present or a gene is not specified. This display will now also pull first from the mappedHgncName property, which is based on a more robust HGNC selection strategy as described in VariantEffect/dcd_mapping2#67.

Gene display improvements:

• Added a new computed property, distinctGenesForTargets, to extract unique mapped HGNC gene names from targetGenes, filtering out empty values.
• Introduced geneTextForScoreSet, which uses distinctGenesForTargets to determine the gene text: if no mapped gene names are available, it falls back to the original gene name; if there is one unique gene, it displays that; if there are multiple, it displays "Multiple genes".
• Updated the gene display in the template to use geneTextForScoreSet, defaulting to "Not specified" if no gene is available.

Related code updates:

• Modified the geneAndYear computed property to use geneTextForScoreSet for consistency in gene name display.