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.

server/main.go

@@ -28,7 +28,7 @@ type Event struct {
 	Type       string  `json:"type"`
 	At         int64   `json:"at"`
 	Note       string  `json:"note"`
-	PhotoID    string  `json:"photoId,omitempty"`
+	PhotoID    string  `json:"photoId,omitempty"` // photo UUIDs, comma-separated (legacy events hold one)
 	Weight     float64 `json:"weight,omitempty"` // kilograms, for "weight" events
 	Grams      float64 `json:"grams,omitempty"`  // food eaten, for "eat" events
 	ExerciseID string  `json:"exerciseId,omitempty"` // for "training" events