fix: Monatswechsel-Markierung – Linien über Events, mehr Abstand, immer waagerecht

Vier Korrekturen:
- Linie verschwand hinter Events: Pseudo-Elemente mit z-index 4 statt
  box-shadow inset, damit Trennlinien immer über den Event-Bars liegen
- Waagerechte Linie auch bei Monatswechsel mitten in einer Zeile (vorher
  nur wenn der Monat am Zeilenanfang begann)
- "1" verschwand hinter Events: cell-day und month-marker bekommen
  z-index 3 + position relative, plus Events-Overlay wird in Zeilen mit
  Monatsmarker um ~26px nach unten geschoben
- Mehr Abstand zwischen Monatskürzel und Trennlinie (padding-top 8px,
  margin-bottom am marker positiv statt negativ)

frontend/css/app.css

@@ -499,17 +499,46 @@ a { color: var(--primary); text-decoration: none; }
 .cell-day.today { background: var(--today-color); color: #fff; font-weight: 700; }
 
 /* Month boundary marker: thicker line + month abbreviation on the 1st */
+.month-col {
+  position: relative; /* anchor for divider pseudo-elements */
+}
 .month-col.first-of-month {
   display: flex;
   flex-direction: column;
   align-items: flex-start;
   gap: 0;
+  padding-top: 8px;
 }
-.month-col.month-divider-left {
-  box-shadow: inset 3px 0 0 0 var(--month-divider-color, #7090c0);
+/* Dividers via pseudo-elements so they render above events (z-index 2) */
+.month-col.month-divider-left::before {
+  content: '';
+  position: absolute;
+  left: 0; top: 0; bottom: 0;
+  width: 3px;
+  background: var(--month-divider-color, #7090c0);
+  z-index: 4;
+  pointer-events: none;
+}
+.month-col.month-divider-top::after {
+  content: '';
+  position: absolute;
+  top: 0; left: 0; right: 0;
+  height: 3px;
+  background: var(--month-divider-color, #7090c0);
+  z-index: 4;
+  pointer-events: none;
 }
 .month-row.month-divider-top {
-  box-shadow: inset 0 3px 0 0 var(--month-divider-color, #7090c0);
+  position: relative;
+}
+.month-row.month-divider-top::before {
+  content: '';
+  position: absolute;
+  top: 0; left: 0; right: 0;
+  height: 3px;
+  background: var(--month-divider-color, #7090c0);
+  z-index: 4;
+  pointer-events: none;
 }
 .month-marker {
   font-size: 14px;
@@ -519,7 +548,19 @@ a { color: var(--primary); text-decoration: none; }
   color: var(--month-label-color, #7090c0);
   line-height: 1;
   padding: 0 2px;
-  margin-bottom: -2px;
+  margin-bottom: 2px;
+  position: relative;
+  z-index: 3; /* above events overlay (z-index 2) */
+}
+/* Make sure the day number stays visible above events */
+.cell-day {
+  position: relative;
+  z-index: 3;
+}
+/* Push events overlay down when row contains a first-of-month cell so the
+   day "1" (which sits below the month marker) isn't hidden by event bars */
+.month-row.has-month-marker .month-events-overlay {
+  top: 56px;
 }
 /* Events overlay — pointer-events:none so clicks pass to columns */
 .month-events-overlay {

frontend/js/views/month.js

@@ -120,6 +120,13 @@ export function renderMonth(container, currentDate, events, onDayClick, onEventC
         style="left:${((c / 7) * 100).toFixed(3)}%;width:${(100 / 7).toFixed(3)}%;bottom:2px">${t('more_events', { n: count })}</div>`;
     });
 
+    // Detect a month boundary in this row. monthChangeIdx is the index of
+    // the first-of-month cell, or -1 if the row doesn't span a month change.
+    let monthChangeIdx = -1;
+    rowCells.forEach((cell, idx) => {
+      if (cell.getDate() === 1 && idx > 0) monthChangeIdx = idx;
+    });
+
     // Full-height column divs (click targets + borders)
     const monthsShort = t('months_short');
     let colsHtml = '';
@@ -134,9 +141,11 @@ export function renderMonth(container, currentDate, events, onDayClick, onEventC
       // First-of-month marker: show month abbreviation, push day number below
       const isFirstOfMonth = cell.getDate() === 1;
       const firstCls   = isFirstOfMonth ? 'first-of-month' : '';
-      // Add divider class on the cell BEFORE a month change (for right border styling)
-      // and on the cell AT a month change (for left border styling) — except at row start
-      const dividerCls = (isFirstOfMonth && idx > 0) ? 'month-divider-left' : '';
+      // Vertical line at month change, horizontal line spanning from change to end of row
+      const dividerClasses = [];
+      if (isFirstOfMonth && idx > 0) dividerClasses.push('month-divider-left');
+      if (monthChangeIdx > 0 && idx >= monthChangeIdx) dividerClasses.push('month-divider-top');
+      const dividerCls = dividerClasses.join(' ');
       const monthLabel = isFirstOfMonth
         ? `<div class="month-marker">${monthsShort[cell.getMonth()]}</div>`
         : '';
@@ -146,9 +155,13 @@ export function renderMonth(container, currentDate, events, onDayClick, onEventC
       </div>`;
     });
 
-    // If the row starts on the 1st of a new month, draw a divider above the row
+    // If the row starts on the 1st of a new month, draw a full-width divider above the row
     const rowDividerCls = rowCells[0].getDate() === 1 ? 'month-divider-top' : '';
-    bodyHtml += `<div class="month-row ${rowDividerCls}">
+    // If any cell in the row is first-of-month, push events overlay down so the day
+    // number isn't hidden by spanning event bars
+    const hasMonthMarker = rowCells.some(c => c.getDate() === 1);
+    const rowMarkerCls = hasMonthMarker ? 'has-month-marker' : '';
+    bodyHtml += `<div class="month-row ${rowDividerCls} ${rowMarkerCls}">
       <div class="month-kw-cell">${kw}</div>
       <div class="month-row-right">
         ${colsHtml}