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/web/sparkline/testdata/chart_empty.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 600 220" class="repo-trend-chart" role="img" aria-label="repo size and snapshot count over time"><line x1="56" y1="110" x2="544" y2="110" stroke="currentColor" stroke-opacity="0.15" stroke-dasharray="3,3"/><text x="300" y="114" text-anchor="middle" font-size="12" fill="currentColor" fill-opacity="0.4">no data yet</text></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 600 220" class="repo-trend-chart" role="img" aria-label="repo size and snapshot count over time"><line x1="72" y1="110" x2="544" y2="110" stroke="currentColor" stroke-opacity="0.15" stroke-dasharray="3,3"/><text x="300" y="114" text-anchor="middle" font-size="12" fill="currentColor" fill-opacity="0.4">no data yet</text></svg>