calendar: include Bis: in the appointment-create template

CalDavSync already reads **Bis:** out of the body on push (and writes
it on pull), but the in-app template used when the user clicks a
calendar day to create an appointment didn't offer the field —
forcing the user to type the marker from scratch for multi-day or
ranged events. Drop it in next to **Zeit:** so the round-trip is
symmetric.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/js/modules/calendar.js

@@ -493,7 +493,7 @@ function openDayPopup(ds, cellEl) {
                 const cfg = calSlug ? getCalConfig(calSlug) : null;
                 const col = cfg?.color || '#fb923c';
                 const calTag = calSlug ? ` #${calSlug}` : '';
-                const body = `**Zeit:** \n**Ort:** \n**Teilnehmer:** \n\n---\n\n**Notizen:**\n\n\n\n#termin${calTag}`;
+                const body = `**Zeit:** \n**Bis:** \n**Ort:** \n**Teilnehmer:** \n\n---\n\n**Notizen:**\n\n\n\n#termin${calTag}`;
                 window._openModal({ id: null, title: `${ds} — `, body, color: col, bucket_id: null });
             } else if (type === 'birthday') {
                 const body = `#geburtstag`;