Add post and event cards, shared calendar sheet and per-event ICS; rework past events and next-event card

Elternbeirat.Web/Features/Events/CalendarSheet.razor.css

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+/* The calendar sheet: a green band with the month over a large day number and the
+   weekday. Fixed width, so the sheets of a list line up whatever the month. The band
+   has its own token (--color-calendar-band): the plain green accent is too light for
+   small white text, the band's darker green keeps it above AA in both schemes.
+   Positioning (gap to the text beside it) is the caller's job. */
+.calendar-sheet {
+    flex-shrink: 0;
+    display: flex;
+    flex-direction: column;
+    width: 4rem;
+    overflow: hidden;
+    border: 1px solid var(--color-border);
+    border-radius: var(--radius-sm);
+    background: var(--color-bg);
+    text-align: center;
+    line-height: 1.2;
+}
+
+.calendar-month {
+    padding: 0.15rem 0;
+    background: var(--color-calendar-band);
+    color: var(--color-on-button);
+    font-size: 0.8rem;
+    font-weight: 700;
+    letter-spacing: 0.05em;
+    text-transform: uppercase;
+}
+
+.calendar-day {
+    padding-top: var(--space-1);
+    color: var(--color-heading);
+    font-size: 1.75rem;
+    font-weight: var(--font-weight-heading);
+}
+
+.calendar-weekday {
+    padding-bottom: var(--space-1);
+    color: var(--color-text-soft);
+    font-size: 0.8rem;
+    font-weight: 600;
+}