Horizontal scroll-shadow cue (#25)

Motivation

Wide tables (the taxonomy Organism table, the Descriptions hits table) scroll horizontally with overflow-x: auto but gave no visual hint that more columns existed off-screen.

User-facing change

• A reusable ScrollShadow wrapper shows a subtle edge fade on whichever side still has hidden content; the fade disappears once that edge is reached. Applied to the taxonomy Organism table and the Descriptions hits table (the widest, minWidth: 1320) — on the latter only the table is wrapped, not the outfmt-gap hint note above it. Respects prefers-reduced-motion.

Code change summary

• web/src/components/ScrollShadow.tsx: new component — owns the overflow container, toggles --at-start / --at-end classes from scroll geometry (scroll listener + ResizeObserver).
• web/src/theme/glass.css: .scroll-shadow edge-fade styles (gradient from --bg-secondary to transparent), opacity toggled by the start/end classes, reduced-motion guard.
• web/src/pages/blastResults/analytics/TaxonomyPanel.tsx: wrapped the Organism table in ScrollShadow.
• web/src/pages/blastResults/analytics/BlastHitsTable.tsx: wrapped the Descriptions table in ScrollShadow (outer container loses its own overflow-x; the hint note stays full-width above it).

Validation evidence

• cd web && npm run build → clean.
• cd web && npm test -- --run TaxonomyPanel → 5 passed.
• npx eslint on the changed files → clean.

No backend / API / IaC changes.