Support sending gallery messages behind a feature flag

Adds the ability to send a gallery message (multiple attachments in one message)
via the media upload preview, wired through the timeline controller/proxy to the
SDK's sendGallery. Gated by a `galleryEnabled` developer flag (default off),
matching Element X Android — with the flag off, attachments are sent as
individual messages as before.

Extends the community PR #5601, rebased onto develop. Tracking:
element-hq/element-meta#3201

Co-authored-by: svetoslav.mitov <svetoslav.mitov@sportinno.com>

ElementX/Sources/Screens/Settings/DeveloperOptionsScreen/View/DeveloperOptionsScreen.swift

@@ -97,6 +97,11 @@ struct DeveloperOptionsScreen: View {
                         .foregroundStyle(.compound.textCriticalPrimary)
                 }
                 
+                Toggle(isOn: $context.galleryEnabled) {
+                    Text("Gallery messages")
+                    Text("Allows sending multiple media in a single message. Received galleries always render regardless of this setting.")
+                }
+                
                 Toggle(isOn: $context.jumpToReadMarkerEnabled) {
                     Text("Jump to unread")
                     Text("Adds a button to jump to the read marker, plus a presence dot on the scroll-to-bottom button when new messages arrive while scrolled away.")