Add SwiftCharts statistics overhaul (iOS 16+)

[AnthonyH93]

Summary

• Replaces custom bar charts on the Statistics tab with SwiftCharts (iOS 16+), with the existing custom charts as a fallback for iOS 14/15
• Adds three new full-screen chart views: Cycling Charts (bar chart with period picker + vs-last-period comparison), Activity Heatmap (calendar-style ride frequency/distance/time grid), and Speed Trend (per-ride line chart)
• Period picker covers 1W / 1M / 3M / 6M / YTD / 1Y across all chart types; last-selected period and heatmap metric are persisted via @AppStorage

New files

• Model/ChartPeriod.swift, Model/ChartDataPoint.swift
• ViewModel/Statistics/StatisticsChartsViewModel.swift
• View/Statistics/CyclingChartsDetailView.swift, CyclingBarChart.swift
• View/Statistics/ActivityHeatmapDetailView.swift, ActivityHeatmapChart.swift
• View/Statistics/SpeedTrendDetailView.swift, SpeedTrendChart.swift

Test plan

• Build on iOS 16+ simulator → Statistics tab shows 3 new chart rows
• Cycling Charts: period picker animates between all 6 periods, metric picker (Distance / Time / Routes) switches data, Δ% comparison strip updates
• Activity Heatmap: metric picker (Routes / Distance / Time) changes heatmap colors, sqrt-scaled vibrancy, last metric persisted across navigation
• Speed Trend: one point per ride, period filter narrows the range, dots hidden when > 20 rides
• Simulate iOS 14/15 → legacy 3-row bar chart path works unchanged
• Theme color applies to all chart components
• Metric/imperial preference applies to distance and speed values

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