Feature: Tagebuch Cover-Bild (Favorit-Funktion) für diary_media

- Migration: diary_media.is_cover (INTEGER DEFAULT 0)
- Upload: erstes Item eines Eintrags automatisch is_cover=1
- Neuer Endpoint: PATCH /diary/{id}/media/{mid}/cover
- GET-Endpoints geben is_cover + cover_url zurück
- Frontend: Stern-Button (⭐) in Gallery-Detail und Edit-Formular
- Timeline-Karte verwendet cover_url als Vorschaubild
- SW by-v212, APP_VER 186

backend/static/css/components.css

@@ -1149,6 +1149,40 @@ html.modal-open {
   font-size:        3rem;
 }
 
+/* Cover-Stern-Button auf Gallery- und Thumbnail-Items */
+.diary-gallery-wrap {
+  position: relative;
+  display:  inline-block;
+}
+.diary-cover-btn {
+  position:         absolute;
+  bottom:           var(--space-1);
+  left:             var(--space-1);
+  width:            28px;
+  height:           28px;
+  border-radius:    50%;
+  border:           none;
+  background:       rgba(0,0,0,.50);
+  color:            rgba(255,255,255,.55);
+  font-size:        14px;
+  cursor:           pointer;
+  display:          flex;
+  align-items:      center;
+  justify-content:  center;
+  padding:          0;
+  line-height:      1;
+  z-index:          2;
+  transition:       color .15s, background .15s;
+}
+.diary-cover-btn--active {
+  color:      #f5c518;
+  background: rgba(0,0,0,.65);
+}
+.diary-cover-btn--form {
+  bottom: var(--space-1);
+  left:   var(--space-1);
+}
+
 /* Card Body */
 .diary-card-body {
   padding: var(--space-3) var(--space-4);