Support multiple photos per event

photoId now holds one or more photo UUIDs, comma-separated. The server
never interprets the field (photos are uploaded and served individually
by UUID), so no schema change is needed and legacy single-photo events
are already valid one-element lists.

Both dialogs let you keep adding photos, previewed as thumbnails with a
per-photo remove button; the file input allows multi-select and no
longer forces the camera, so the gallery is available too. History rows
show every photo, each opening in the lightbox.

src/style.css

@@ -432,13 +432,33 @@ dialog menu {
 .photo-preview {
   margin-top: 8px;
   width: 100%;
+  display: flex;
+  flex-wrap: wrap;
+  gap: 10px;
 }
-.photo-preview img {
+.photo-preview[hidden] { display: none; }
+.photo-thumb { position: relative; }
+.photo-thumb img {
   display: block;
-  max-width: 100%;
-  max-height: 240px;
+  width: 76px;
+  height: 76px;
+  object-fit: cover;
   border-radius: 8px;
   border: 1px solid var(--border);
+  background: var(--accent-soft);
+}
+.photo-thumb-remove {
+  position: absolute;
+  top: -7px;
+  right: -7px;
+  width: 22px;
+  height: 22px;
+  padding: 0;
+  border-radius: 50%;
+  background: var(--danger);
+  color: #fff;
+  font-size: 14px;
+  line-height: 1;
 }
 
 .event .thumb {