[FRONTEND] Make analytics charts interactive with hover tooltips

[dev-fatima-24]

Priority: medium

Description

Charts on the analytics pages are currently static SVGs. Adding hover tooltips that display exact values for each data point makes the charts more useful for issuers reviewing vaccination trends.

Acceptance Criteria

• Hovering a data point shows a tooltip with label and value
• Tooltip is positioned to avoid clipping at chart edges
• Tooltip disappears on mouse leave
• Keyboard users can focus data points and trigger tooltips
• Tooltip content is announced by screen readers via aria-live