feat: Emit ImexProgress(1) after receiving backup size

UIs may want to display smth like "Transferring..." after "Establishing connection between
devices..." on nonzero progress. Before, progress on the receiver side was starting with 2 after
receiving enough data.

src/imex/transfer.rs

@@ -178,6 +178,7 @@ impl BackupProvider {
         }
 
         info!(context, "Received valid backup authentication token.");
+        // Emit a nonzero progress so that UIs can display smth like "Transferring...".
         context.emit_event(EventType::ImexProgress(1));
 
         let blobdir = BlobDirContents::new(&context).await?;
@@ -309,6 +310,10 @@ pub async fn get_backup2(
     let mut file_size_buf = [0u8; 8];
     recv_stream.read_exact(&mut file_size_buf).await?;
     let file_size = u64::from_be_bytes(file_size_buf);
+    info!(context, "Received backup file size.");
+    // Emit a nonzero progress so that UIs can display smth like "Transferring...".
+    context.emit_event(EventType::ImexProgress(1));
+
     import_backup_stream(context, recv_stream, file_size, passphrase)
         .await
         .context("Failed to import backup from QUIC stream")?;
@@ -434,12 +439,14 @@ mod tests {
         assert!(msg.save_file(&ctx1, &path).await.is_err());
 
         // Check that both received the ImexProgress events.
-        ctx0.evtracker
-            .get_matching(|ev| matches!(ev, EventType::ImexProgress(1000)))
-            .await;
-        ctx1.evtracker
-            .get_matching(|ev| matches!(ev, EventType::ImexProgress(1000)))
-            .await;
+        for ctx in [&ctx0, &ctx1] {
+            ctx.evtracker
+                .get_matching(|ev| matches!(ev, EventType::ImexProgress(1)))
+                .await;
+            ctx.evtracker
+                .get_matching(|ev| matches!(ev, EventType::ImexProgress(1000)))
+                .await;
+        }
     }
 
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]