ui: chart polish — rotated y-axis labels, wider viewBox, single-day fallback

- Add rotated 'Size' (left) and 'Snapshots' (right) axis titles in
  the chart's outer margins so the two y-axes are self-describing.
- Bump the chart viewBox from 600x220 to 640x220 and lift padL from
  56 to 72 so the rotated labels and byte tick numbers don't crowd.
- Dedupe the X-axis labels for short windows (1 or 2 days collapsed
  the start/mid/end indices onto each other, stacking 'May 7' three
  times); the 1-day case now centres a single label, 2-day uses
  start+end only.
- Pin a lone data dot to the chart centre instead of the left edge
  when len(days)==1, so it sits under the centred date label.

Goldens regenerated.

internal/server/http/ui_repo.go

@@ -284,7 +284,7 @@ func (s *Server) buildRepoTrendView(ctx context.Context, hostID, rangeKey string
 	chartSVG := sparkline.RenderChart([]sparkline.Series{
 		{Name: "size", Stroke: "#3b82f6", Axis: sparkline.AxisLeft, Format: sparkline.FormatBytes, Points: sizes},
 		{Name: "snapshots", Stroke: "#f59e0b", Axis: sparkline.AxisRight, Format: sparkline.FormatCount, Points: counts},
-	}, dayList, sparkline.ChartOpts{Width: 600, Height: 220})
+	}, dayList, sparkline.ChartOpts{Width: 640, Height: 220})
 	return repoTrendView{HostID: hostID, Range: rangeKey, ChartSVG: chartSVG}
 }