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feat: Metrics tab with interactive time-series chart (zoom + level colors)

Fill the empty Metrics section with a Grafana-style log-volume histogram over time, stacked by level (Information/Warning/Error colours). Drag-select a region on the chart to zoom into that time range; Back/Reset walk the zoom history.

• Core: TimeHistogram/TimeBucket models; GetTimeHistogramAsync on ILogIndex/ILogQueryService
• Infrastructure: in-memory bucketizer; Lucene histogram via a docvalues collector (ts NumericDocValues + level SortedDocValues), range auto-computed from min/max when not zoomed
• App: TimeHistogramChart (SkiaSharp, drag-select -> SelectRangeCommand), MetricsViewModel (zoom stack, summary counts), wired into MainViewModel; refreshes on import, lazy-loads on first open