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.

README.md

@@ -51,8 +51,9 @@ archive), who is on call, the alert feed, and an *Account* tab for your own
 password and the ntfy topic your pages go to. It is built for a phone first. On a phone
 it navigates through a hamburger menu and has a sticky action bar, it follows the
 system's dark mode, and it can be added to the home screen. From 900px wide it switches
-to a sidebar with the queue and the incident side by side. Statistics remain in
-[terdut-tui](https://github.com/yeniklas/terdut-tui) for now.
+to a sidebar with the queue and the incident side by side. The Stats page shows
+incident counts, MTTA and MTTR, and alert frequency by name, hour and day over a
+chosen range.
 
 You sign in with a username and password. Users have no password until one is
 set, and a user without one can only use API keys: