Stats heatmap: empty hours same height; cname_chain only for long chains

The "Busiest times" grid's hours without data used the class "empty",
which picked up the padding of empty messages. One CNAME to another no
longer warns; three or more get an info note showing the chain.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/static/assets/style.css

@@ -398,7 +398,7 @@ td.num, th.num { text-align: right; font-variant-numeric: tabular-nums; }
 .hm-row { display: flex; align-items: center; }
 .hm-cell { height: 22px; border-radius: 3px; }
 .hm-cell:hover, .hm-cell:focus { outline: 2px solid var(--text); outline-offset: -1px; }
-.hm-cell.empty, .hm-swatch.empty { background: transparent; box-shadow: inset 0 0 0 1px var(--chart-grid); }
+.hm-empty { background: transparent; box-shadow: inset 0 0 0 1px var(--chart-grid); }
 .heat-0 { background: var(--heat-0); } .heat-1 { background: var(--heat-1); } .heat-2 { background: var(--heat-2); }
 .heat-3 { background: var(--heat-3); } .heat-4 { background: var(--heat-4); } .heat-5 { background: var(--heat-5); }
 .heat-6 { background: var(--heat-6); }