Files · Transfers: file operations, a transfer queue, downloads and storage

A file on its way is a Transfer with a state: waiting, moving at a rate,
paused, done or failed with a reason. One row draws it for the queue and
the downloads: its icon, its way, a bar while it moves or waits paused, a
status with the time left, and only the actions its state allows.
TransferQueue counts what moves and waits. DownloadManager keeps the
finished below under Clear, opening on a press and showing their folder.
FileOperationProgress tells a copy or move with its bar, time left and
current file, its header wrapping by the header rule; a failure speaks in
the danger tone. StorageUsage points to data_display's UsageBar.

tasks/ch21-30.md

@@ -315,10 +315,10 @@
 
 ## T27d Files · Transfers
 
-- [ ] FileOperationProgress
-- [ ] TransferQueue
-- [ ] DownloadManager
-- [ ] StorageUsage → `data_display::UsageBar`
+- [x] FileOperationProgress
+- [x] TransferQueue
+- [x] DownloadManager
+- [x] StorageUsage → `data_display::UsageBar`
 
 ## T28 Messaging — `src/messaging`