feat: overlapping event layout and calendar event attachments (#107)

- Overlapping timed events in week/day views now render side-by-side using a column-layout algorithm
- Calendar events support optional file attachments (images, PDFs, documents up to 5 MB)
- Attachment images shown in event popup; other files as download links
- Birthday modal redesigned with photo/avatar side-by-side with name/date fields
- DB migration 27: adds attachment_name, attachment_mime, attachment_size, attachment_data columns to calendar_events
- Server-side MIME allowlist and size validation for attachments
- i18n: all 15 locales include new attachment keys (de properly translated)

Co-Authored-By: Rafael Foster <rafaelfoster@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/styles/calendar.css

@@ -391,8 +391,6 @@
 
 .week-event {
   position: absolute;
-  left: 2px;
-  right: 2px;
   border-radius: var(--radius-xs);
   padding: var(--space-0h) var(--space-1);
   font-size: var(--text-xs);
@@ -424,6 +422,29 @@
   opacity: 0.85;
 }
 
+.event-attachment-preview {
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-2);
+  margin-top: var(--space-2);
+}
+
+.event-attachment-preview img {
+  width: 100%;
+  max-height: 180px;
+  object-fit: cover;
+  border-radius: var(--radius-md);
+  border: 1px solid var(--color-border-subtle);
+}
+
+.event-attachment-preview a {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-2);
+  color: var(--color-accent);
+  word-break: break-word;
+}
+
 /* --------------------------------------------------------
  * Tagesansicht
  * -------------------------------------------------------- */
@@ -768,6 +789,25 @@
   border-top: 1px solid var(--color-border);
 }
 
+.event-popup__attachment {
+  margin-top: var(--space-2);
+}
+
+.event-popup__attachment--image img {
+  width: 100%;
+  max-height: 220px;
+  object-fit: cover;
+  border-radius: var(--radius-md);
+  border: 1px solid var(--color-border-subtle);
+}
+
+.event-popup__attachment--file {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--space-2);
+  color: var(--color-accent);
+}
+
 /* --------------------------------------------------------
  * Ganztägige Ereignisse (oben in Wochen-/Tagesansicht)
  * -------------------------------------------------------- */