fix(a11y): remove tabIndex on BarChart bars inside aria-hidden

[dezzy-music]

Summary

Fixes #708.

BarChart's visual bars container is marked aria-hidden, but individual bars inside had tabIndex={0}. This violates axe's aria-hidden-focus rule because screen reader users tabbing through the page land on silent tab stops that assistive technology is told do not exist.

Changes

• Removed tabIndex={0} and keyboard focus-visible outline classes from the decorative bars in src/components/ui/BarChart.tsx.
• Visual hover tooltip is preserved via group-hover:opacity-100.
• Assistive technology and keyboard screen readers continue to access all data via the visually-hidden (sr-only) accessible data table.
• Updated unit tests in BarChart.test.tsx to assert that no focusable elements exist inside the aria-hidden container.

Verification

• Ran Jest: 8/8 tests pass in BarChart.test.tsx and 3/3 tests pass in dashboard-charts.a11y.test.tsx.
• Ran ESLint: 0 errors.

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