Login page for AUTH_MODE=local instead of the Basic Auth popup

The local mode now shows a login page in the colors of the UI (light/dark theme, FR/EN)
with a signed session cookie and the log out button, like the OIDC mode. LOGIN_LOGO shows
a PNG mounted in the container on that page. SESSION_TTL applies to both modes
(OIDC_SESSION_TTL still works). HTTP Basic credentials are still accepted for scripts.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

web/app.js

@@ -2103,7 +2103,7 @@ $('#range').value = store.get('range', '1h');
 if (!$('#range').value) $('#range').value = '1h';
 $('#severity').value = store.get('severity', '');
 
-// With OIDC login, show who is logged in and the log out button.
+// With a login (local or OIDC), show who is logged in and the log out button.
 fetch('/auth/me').then((res) => (res.ok ? res.json() : null)).then((me) => {
   if (!me || !me.user) return;
   const btn = $('#logoutBtn');