feat: kpi chart improvements

[briangregoryholmes]

This PR addresses a few limitations of the KPI/Chart sparkline component regarding tooltips and comparison data:

1. Lines with different numbers of points became out of sync (e.g., 31 days vs 30 days)
2. Tooltip trigger area was inconsistent and hard to activate
3. Tooltip showed empty when hovering over null data points
4. Comparison mode didn't show timestamps or "vs." dates

Comparison data sync

• Replaced date-based x-axis positioning with index-based positioning
• Both lines now align by index (day 1 vs day 1) regardless of actual dates
• Uses the longer of the two datasets for the x-scale, so no data is truncated

Tooltip improvements

• Tooltip now triggers reliably based on x-position (removed restrictive y-proximity check)
• Fixed empty tooltip container showing when all points at an index are null
• Added formatted timestamps to tooltip when comparison mode is active
• Tooltip is tracked to the cursor position

Range display

• Shows "vs." comparison date when hovering (e.g., "May 30 vs. Apr 30, 2025")
• Static date labels correctly show primary data's actual date range

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!