fix(dashboard): savings-trend tooltip omits year for daily/weekly intervals; ambiguous across year boundaries

[cristim]

Symptom

After LeanerCloud/cloud-commitments-cli#1257 the Home savings-over-time tooltip shows a date instead of the raw 13-digit millisecond value (QA 3.2). However, the helper it reuses (formatTrendAxisTick in frontend/src/dashboard.ts:996) returns "Jun 15" for daily/weekly intervals with no year component. The hourly branch also omits the year.

For the 90d range (daily) and the new ~365-day All range (weekly), the tooltip can show two points labeled "Jun 15" -- one from this year and one from last year -- with no way to distinguish them. The same is true for any range straddling a year boundary (e.g. clicking 90d in February).

Root cause

The frontend reuses formatTrendAxisTick for both x-axis tick labels and the tooltip title (PR LeanerCloud/cloud-commitments-cli#1257). The axis-tick branch needs to be compact (limited horizontal space per label), but the tooltip has plenty of room and would benefit from including the year.

export function formatTrendAxisTick(tsMs: number, intervalHint: 'hourly' | 'daily' | 'weekly'): string {
  const d = new Date(tsMs);
  if (intervalHint === 'hourly') {
    return d.toLocaleString('en-US', { month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', hour12: false });
  }
  return d.toLocaleDateString('en-US', { month: 'short', day: 'numeric' });  // no year
}

Fix

Add a separate formatter formatTrendTooltipTitle(tsMs, interval) that includes the year. The axis-tick formatter stays as-is (compact). Wire the tooltip title callback to the new helper:

export function formatTrendTooltipTitle(tsMs: number, intervalHint: 'hourly' | 'daily' | 'weekly'): string {
  const d = new Date(tsMs);
  if (intervalHint === 'hourly') {
    return d.toLocaleString('en-US', { year: 'numeric', month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit', hour12: false });
  }
  return d.toLocaleDateString('en-US', { year: 'numeric', month: 'short', day: 'numeric' });
}

Test parity: extend the existing tooltip-title test in dashboard.test.ts to assert the rendered string contains the year (/2024/).

Notes

• Locale stays en-US (consistent with the rest of the chart). A separate i18n issue can revisit if/when CUDly localizes.
• Timezone handling (UTC bucket vs local display) is the broader concern tracked elsewhere (see fix(ui): enforce future-only planned start date + widen Configure Purchase Term field cloud-commitments-cli#1258 follow-ups); not in scope here.

Discovered via

Adversarial review of LeanerCloud/cloud-commitments-cli#1257.