Add rolling-average trend line to insulin statistics charts

[Sjoerd-Bo3]

Summary

Adds a rolling-average (moving average) trend line overlaid on the insulin bar charts in Statistics → Insulin, so longer-term trends are easy to read on top of the noisy per-bar values — similar to the trend line in the attached reference example.

The dashed line is added to both charts in the Insulin tab:

• Total Daily Dose — rolling average of the daily/hourly TDD.
• Bolus Insulin — rolling average of the total bolus (manual + SMB + external), tracking the top of the stacked bars.

What changed

• StatChartUtils: new reusable helpers
  • rollingAverage(for:date:value:window:centerOffset:) — a centered moving average with clamped partial windows at the edges, so the line spans the full visible range and isn't shifted relative to the bars.
  • rollingAverageWindow(for:) — interval-aware window (3 hours for Day; 3 days for Week; 7 days for Month/3M).
  • barCenterOffset(for:) — re-centers the line on bar centers (bars are binned to the start of the hour/day).
  • dashedLegendItem(label:color:) + a small HorizontalLine shape for a dashed legend symbol.
  • Shared rollingAverageStrokeStyle.
• TotalDailyDoseChart: overlay the dashed LineMark (catmullRom interpolation) and add a legend (the chart previously had none).
• BolusStatsView: overlay the dashed LineMark over the total bolus and add a "Rolling average" entry to the existing legend.

The line uses Color.primary, so it stays high-contrast in both light and dark mode.

Notes / design choices

• Centered vs trailing average: centered keeps the line aligned with the bars it summarizes (no lag), at the cost of the last point not being a "pure" trailing average — appropriate for a retrospective statistics view.
• Window sizes are heuristic; happy to tune them.
• No data-layer changes — the average is computed in the view from the already-fetched stats arrays.

Does this make sense for the other Statistics tabs?

• Meals (Macro Nutrients) — yes, this is the most natural next candidate; carb/macro intake is bursty day-to-day and a trend line reads well. Trivial to add with the same helper (rolling average of carbs, or per-macro).
• Glucose — the Glucose tab already shows trend-style summaries (percentiles, TIR, distributions) rather than a single per-day bar series, so a rolling-average line doesn't map cleanly onto those charts. (A daily-average-glucose line would be a separate, larger feature.)
• Looping — the loop chart is a horizontal share/percentage bar (success vs glucose count) without a time series, so a rolling average doesn't apply.

If you'd like, I can extend the trend line to the Meals chart in this same PR.

Test plan

• Build and open Statistics → Insulin.
• Verify the dashed trend line renders over both the Total Daily Dose and Bolus Insulin charts across D / W / M / 3M.
• Confirm the line aligns with bar centers and the legend shows "Rolling average".
• Confirm light/dark mode contrast.

🤖 Generated with Claude Code

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