Keep downloads inside the folder they were sent to, never run a file opened for editing, and let a bucket upload be retried

- A remote name went straight into Path.Combine. A POSIX name may hold backslashes, a colon or a drive
  letter, and an S3 key may hold "../", so a name chosen by whoever could write to the host decided
  where on this machine a download landed. LocalDirectory.ChildPath cleans the name and requires the
  resolved parent to be the target folder; every download path and EDIT use it, and a name that cannot
  be a file there is left on the host and reported.
- EDIT handed the copy to the shell's default action, which for .bat, .hta, .js and friends is to run
  it. LocalEditor opens such a file with its own "edit" verb or Notepad on Windows and `open -t` on
  macOS, and refuses a .desktop launcher on Linux.
- A failed bucket upload leaves its part object, and a retry asked for an offset no bucket accepts, so
  the row could never finish. IRemoteFileStore.CanWriteFromAnOffset is false for buckets and the queue
  starts those over.
- The SFTP host picker re-found its selection from the empty moment of every vault reload and landed
  on the first host once a minute; the chosen host is now remembered, and a typed password is only
  sent to the host it was typed for.

src/DodoSSH.Client.Ssh/SftpSession.cs

@@ -523,6 +523,14 @@ public interface IRemoteFileStore : IAsyncDisposable
     /// <param name="cancellationToken">Abandons the open.</param>
     Task<Stream> OpenWriteAsync(string path, long offset, CancellationToken cancellationToken);
 
+    /// <summary>Whether <see cref="OpenWriteAsync"/> can start anywhere but the beginning.</summary>
+    /// <remarks>
+    /// True for SFTP, which is a file system. False for a bucket, where an object is written whole: an
+    /// interrupted upload there starts again rather than resuming, and the transfer queue reads this to know
+    /// — asking for an offset would be refused on every retry, leaving a row that can never finish.
+    /// </remarks>
+    bool CanWriteFromAnOffset => true;
+
     /// <summary>Creates one directory, whose parent must exist.</summary>
     Task CreateDirectoryAsync(string path, CancellationToken cancellationToken);