Handle data outage periods with smoothing and visual indicators

Follow-up from PR #52, related to parent issue #61

Current situation

• Data outage occurred from 2024-01-24 to 2025-06-09
• Currently marked with ReferenceArea components showing "Data outage"
• Gaps in data make trends hard to read

Proposed solution

1. Data interpolation

• Smooth/interpolate missing data during outage periods
• Use linear interpolation or moving average from surrounding data points
• Store interpolated values with a flag indicating they're estimates

2. Visual indicators

• Keep the gray ReferenceArea overlays to show estimated periods
• Consider adding a note in chart subtitle about estimated data
• Maybe use dashed lines or different opacity for interpolated data points

Acceptance criteria

• Data gaps during outage periods are filled with reasonable estimates
• Visual indicators clearly show which periods contain estimated data
• Charts show smooth trends without jarring gaps
• Legend or note explains that some data is interpolated

Start by reviewing PR #52 and parent issue #61, then inspect the existing chart handling for ReferenceArea overlays and data gaps. Determine how outage-period estimates and their flags should flow into the charts. Done means gaps are filled with reasonable estimates, estimated periods remain visibly marked, trends are smooth, and the chart legend or subtitle explains the interpolation.