Fold the stats pages to fit a phone

The stats pages were laid out for a desktop width and did not fit a phone.
Below 600px the top-alerts rows now take three lines (name, count and
sparkline, last seen and mean fix) instead of six columns, the heatmap
shrinks to the screen with fewer hour labels, and the drilldown's
time-to-resolve bars and occurrences table fold into stacked lines and
cards rather than scrolling sideways. Grid children on the stats pages may
now shrink below their content width, which is what let the heatmap push
the page wider than the screen.

CSS and one data-label attribute only: no API, schema or behaviour change
at wider widths.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

internal/web/static/js/statsalert.js

@@ -103,17 +103,20 @@ function fixBars(d) {
   }));
 }
 
+// On a phone each row folds into a card; the labels come from data-label.
+const OCC_COLS = ['Fired (UTC)', 'Time to ack', 'Duration', 'Acknowledged by', 'Status'];
+
 function table(d) {
   return h('div', { class: 'occ-wrap' },
     h('table', { class: 'occ' },
-      h('thead', {}, h('tr', {}, ['Fired (UTC)', 'Time to ack', 'Duration', 'Acknowledged by', 'Status'].map((t) => h('th', { text: t })))),
+      h('thead', {}, h('tr', {}, OCC_COLS.map((t) => h('th', { text: t })))),
       h('tbody', {}, d.occurrences.map((o) => h('tr', {},
         h('td', {}, h('a', { href: `/incidents/${o.incident_id}`, text: fmtTime(o.triggered_at) }),
           ` ${WEEKDAYS[new Date(o.triggered_at * 1000).getUTCDay()]}`),
-        h('td', { text: sec(o.ack_seconds) }),
-        h('td', { text: sec(o.resolve_seconds) }),
-        h('td', { text: o.acknowledged_by || '—' }),
-        h('td', { text: o.status }))))));
+        h('td', { 'data-label': OCC_COLS[1], text: sec(o.ack_seconds) }),
+        h('td', { 'data-label': OCC_COLS[2], text: sec(o.resolve_seconds) }),
+        h('td', { 'data-label': OCC_COLS[3], text: o.acknowledged_by || '—' }),
+        h('td', { 'data-label': OCC_COLS[4], text: o.status }))))));
 }
 
 export function body(name, d, error, onRange) {