Add a Stats page to the web UI

Statistics used to live only in terdut-tui; the account page said so.
The page shows the same figures as the TUI's Stats tab -- incident
counts, MTTA and MTTR, top alerts, and alert frequency by hour (UTC) and
by day of week -- and adds a range picker (Today, 7d, 30d, 90d, All)
that the TUI does not have. The ranges are day-granular because the
server reads from/to as whole UTC dates, so there is no 24h chip.

No server change: the page uses the existing /api/stats/* endpoints,
which already scope to the caller's teams. Charts are inline SVG and
plain elements sized from script, because the CSP forbids inline styles,
inline scripts and CDN libraries.

Removes the "statistics are in terdut-tui" notes from the account page
and the README.

internal/web/static/js/ui.js

@@ -43,6 +43,7 @@ const ICONS = {
   undo: ['M9 14L4 9l5-5', 'M4 9h10a6 6 0 0 1 0 12h-3'],
   user: ['circle:12,8,3.5', 'M5 20a7 7 0 0 1 14 0'],
   clock: ['circle:12,12,9', 'M12 7v5l3 2'],
+  chart: ['M4 20h16M7 20v-7M12 20V6M17 20v-10'],
   bell: ['M6 16V11a6 6 0 0 1 12 0v5l1.5 2h-15z', 'M10 20.5a2 2 0 0 0 4 0'],
   note: ['M5 4h14v12l-4 4H5z', 'M15 20v-4h4', 'M9 9h6M9 13h4'],
   archive: ['M3.5 5h17v4h-17z', 'M5 9v10h14V9', 'M10 13h4'],