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/app.js

@@ -8,6 +8,7 @@ import * as queue from './queue.js';
 import * as incident from './incident.js';
 import * as oncall from './oncall.js';
 import * as alerts from './alerts.js';
+import * as stats from './stats.js';
 import * as account from './account.js';
 import * as team from './team.js';
 import * as admin from './admin.js';
@@ -23,6 +24,7 @@ const SECTIONS = {
   queue: { title: 'Queue', view: queue },
   oncall: { title: 'On-call', view: oncall },
   alerts: { title: 'Alerts', view: alerts },
+  stats: { title: 'Stats', view: stats },
   team: { title: 'Team', view: team },
   admin: { title: 'Admin', view: admin },
   adminuser: { title: 'User', view: adminuser, nav: 'admin' },
@@ -36,6 +38,7 @@ const NAV_ITEMS = [
   { path: '/', section: 'queue', label: 'Queue', icon: 'queueList' },
   { path: '/oncall', section: 'oncall', label: 'On-call', icon: 'calendar' },
   { path: '/alerts', section: 'alerts', label: 'Alerts', icon: 'bell' },
+  { path: '/stats', section: 'stats', label: 'Stats', icon: 'chart' },
   { path: '/team', section: 'team', label: 'Team', icon: 'team' },
   { path: '/admin', section: 'admin', label: 'Admin', icon: 'shield', adminOnly: true },
   { path: '/more', section: 'more', label: 'Account', icon: 'user' },
@@ -58,7 +61,7 @@ function parseRoute(pathname) {
   if (t) return { section: 'admin', tab: t.tab };
   const tt = team.TABS.find((x) => x.path === `/${name}`);
   if (tt) return { section: 'team', tab: tt.tab };
-  if (name === 'oncall' || name === 'alerts' || name === 'more') return { section: name };
+  if (name === 'oncall' || name === 'alerts' || name === 'stats' || name === 'more') return { section: name };
   return { section: 'queue', incident: null };
 }