feat(caldav): username/password (Basic Auth) access with discovery + https URLs

- dav_router: add Basic-Auth principal-discovery tree at /caldav/ (and
  /.well-known/caldav) so clients can add a CalDAV account with server URL +
  username + password; lists all published calendars. Token URL /dav/{token}/
  still works without login. Handlers generalised over a base href.
- dav_util: derive the public origin from X-Forwarded-Proto/-Host (or
  PUBLIC_BASE_URL) so published URLs are https, not internal http:8080.
- local_router: expose caldav_login_url alongside caldav_url.
- frontend/i18n: show both the no-login token URL and the login URL + hint.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/i18n.js

@@ -117,12 +117,14 @@ const translations = {
     copy: 'Kopieren',
     caldav_publish: 'CalDAV veröffentlichen',
     caldav_unpublish: 'CalDAV deaktivieren',
-    caldav_published_url: 'CalDAV-URL:',
+    caldav_published_url: 'CalDAV-URL (ohne Login):',
     caldav_url_copied: 'CalDAV-URL kopiert',
     caldav_token_rotate: 'Token neu generieren',
     caldav_token_rotated: 'Neuer Token erzeugt – alte Abos müssen neu eingerichtet werden',
     caldav_rotate_confirm: 'Neuen Token erzeugen? Die bisherige URL wird ungültig und bestehende Abos müssen mit der neuen URL neu eingerichtet werden.',
-    caldav_hint: 'Jeder mit dieser URL kann diesen Kalender abonnieren und bearbeiten. Über CalDAV-fähige Clients (Apple Kalender, Thunderbird, DAVx5) einbinden.',
+    caldav_hint: 'Jeder mit dieser URL kann diesen Kalender abonnieren und bearbeiten – kein Login nötig. Über CalDAV-fähige Clients (Apple Kalender, Thunderbird, DAVx5) einbinden.',
+    caldav_login_url: 'CalDAV-URL (mit Login):',
+    caldav_login_hint: 'Alternativ im Client ein „CalDAV-Konto" mit dieser Server-URL sowie deinem Benutzernamen und Passwort hinzufügen – dann werden alle deine veröffentlichten Kalender gefunden.',
     share: 'Teilen',
     import: 'Importieren',
     export: 'Exportieren',
@@ -419,12 +421,14 @@ const translations = {
     copy: 'Copy',
     caldav_publish: 'Publish via CalDAV',
     caldav_unpublish: 'Unpublish CalDAV',
-    caldav_published_url: 'CalDAV URL:',
+    caldav_published_url: 'CalDAV URL (no login):',
     caldav_url_copied: 'CalDAV URL copied',
     caldav_token_rotate: 'Regenerate token',
     caldav_token_rotated: 'New token generated – existing subscriptions must be re-added',
     caldav_rotate_confirm: 'Generate a new token? The current URL will stop working and existing subscriptions must be re-added with the new URL.',
-    caldav_hint: 'Anyone with this URL can subscribe to and edit this calendar. Add it in a CalDAV-capable client (Apple Calendar, Thunderbird, DAVx5).',
+    caldav_hint: 'Anyone with this URL can subscribe to and edit this calendar — no login required. Add it in a CalDAV-capable client (Apple Calendar, Thunderbird, DAVx5).',
+    caldav_login_url: 'CalDAV URL (with login):',
+    caldav_login_hint: 'Alternatively add a "CalDAV account" in your client using this server URL plus your username and password — it will discover all your published calendars.',
     share: 'Share',
     import: 'Import',
     export: 'Export',