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/Services/Timeline/TimelineProxy.swift

@@ -374,6 +374,30 @@ final class TimelineProxy: TimelineProxyProtocol {
         return .success(())
     }
     
+    func sendGallery(itemInfos: [GalleryItemInfo],
+                     caption: String?,
+                     formattedCaption: String?,
+                     inReplyToEventID: String?) async -> Result<Void, TimelineProxyError> {
+        MXLog.info("Sending gallery with \(itemInfos.count) items")
+        
+        do {
+            let formatted = formattedCaption.map { FormattedBody(format: .html, body: $0) }
+            let params = GalleryUploadParameters(caption: caption,
+                                                 formattedCaption: formatted,
+                                                 mentions: nil,
+                                                 inReplyTo: inReplyToEventID)
+            
+            let handle = try timeline.sendGallery(params: params, itemInfos: itemInfos)
+            try await handle.join()
+            MXLog.info("Finished sending gallery")
+        } catch {
+            MXLog.error("Failed sending gallery with error: \(error)")
+            return .failure(.sdkError(error))
+        }
+        
+        return .success(())
+    }
+    
     func sendVoiceMessage(url: URL,
                           audioInfo: AudioInfo,
                           waveform: [Float],