fix: share-card ms->min unit; leaderboard 10-row cap; strip screenshot chrome

[Flotapponnier]

Reported on wormhole-vaa-latency:

1. Ranking PNG showed nonsense units: values like 858.79 s printed as "858.79 min" because fmtValue stripped " s" from fmtUnit's output while unitSuffix independently returned " min" at the same threshold. Aligned fmtUnit for ms with unitSuffix: auto-flip to min at 60000 ms.
2. Leaderboard PNG overlap: 14-row bench pushed row 10+ under the CardFooter divider. Hard-capped to 10 rows with an "and N more" line.
3. On-page screenshot download included the header metadata row and Top-N selector — the reader wanted just the plot. Added data-chart-export-omit attribute on both rows and taught the html-to-image filter to drop them.

Test plan

• /benchmarks/wormhole-vaa-latency ranking PNG shows "14.3 min" not "858.79 min" for Ethereum
• Leaderboard PNG shows 10 rows + "and 4 more on openchainbench.com"
• Chart Download PNG button on-page produces bars only (no header, no TopN selector)