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/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-07-13", "text": "Attach multiple photos to an event — the photo picker now also offers the gallery with multi-select" },
   { "date": "2026-07-13", "text": "Track food by weight: logging a meal asks for grams (optional), with a daily total in the overview and a weekly chart" },
   { "date": "2026-07-12", "text": "Browse the full changelog from the bottom of the page" },
   { "date": "2026-07-12", "text": "Finer-grained y-axis on the daily counts chart" },