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.

tests/DodoSSH.Client.App.Tests/LocalEditorTests.cs

@@ -0,0 +1,70 @@
+using DodoSSH.Client.App.Platform;
+
+namespace DodoSSH.Client.App.Tests;
+
+/// <summary>
+/// <c>LocalEditor</c>: a remote file opened for editing is never run.
+/// </summary>
+/// <remarks>
+/// The file's name was chosen by whoever can write to the host, so the question for every name here is
+/// whether the desktop's default action would execute it. Only the Windows rule is exercised directly — it
+/// is a pure function of the name and <c>PATHEXT</c> — while what each platform starts is checked on the
+/// platform the suite is running on.
+/// </remarks>
+public sealed class LocalEditorTests
+{
+    [Theory]
+    [InlineData("deploy.bat")]
+    [InlineData("DEPLOY.CMD")]
+    [InlineData("notes.hta")]
+    [InlineData("setup.js")]
+    [InlineData("link.lnk")]
+    [InlineData("run.ps1")]
+    [InlineData("settings.reg")]
+    [InlineData("deploy.bat.")]
+    [InlineData("deploy.bat ")]
+    public void AFileWindowsWouldRun_IsRecognised(string name) =>
+        LocalEditor.WouldRunOnWindows(name, pathExt: string.Empty).ShouldBeTrue();
+
+    [Theory]
+    [InlineData("nginx.conf")]
+    [InlineData("authorized_keys")]
+    [InlineData(".bashrc")]
+    [InlineData("app.json")]
+    [InlineData("bat")]
+    public void AnOrdinaryConfigFile_IsLeftToItsAssociation(string name) =>
+        LocalEditor.WouldRunOnWindows(name, pathExt: string.Empty).ShouldBeFalse();
+
+    [Fact]
+    public void WhateverPathExtAdds_IsTreatedAsRunnableToo() =>
+        LocalEditor.WouldRunOnWindows("tool.foo", pathExt: ".COM;.EXE;.FOO").ShouldBeTrue();
+
+    [Fact]
+    public void ARunnableFile_IsNeverHandedToTheDefaultOpenAction()
+    {
+        var path = Path.Combine(Path.GetTempPath(), "deploy.bat");
+
+        if (OperatingSystem.IsLinux())
+        {
+            // No portable editor to name, and a .bat is not something a Linux desktop runs.
+            LocalEditor.StartInfoFor(path).UseShellExecute.ShouldBeTrue();
+            Should.Throw<InvalidOperationException>(() => LocalEditor.StartInfoFor("/tmp/app.desktop"));
+            return;
+        }
+
+        var start = LocalEditor.StartInfoFor(OperatingSystem.IsMacOS() ? "/tmp/run.command" : path);
+
+        // Either the type's own "edit" verb, or a named editor with the file as an argument — never the
+        // file itself under its default verb.
+        (start.Verb == "edit" || !start.UseShellExecute).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void AnOrdinaryFile_StillOpensWithItsAssociation()
+    {
+        var start = LocalEditor.StartInfoFor(Path.Combine(Path.GetTempPath(), "nginx.conf"));
+
+        start.UseShellExecute.ShouldBeTrue();
+        start.Verb.ShouldBeEmpty();
+    }
+}

tests/DodoSSH.Client.App.Tests/TransferQueueingTests.cs

@@ -131,6 +131,35 @@ public sealed class TransferQueueingTests : IDisposable
     /// SFTP fake that a real connect produces.
     /// </para>
     /// </remarks>
+    /// <summary>
+    /// A name the host chose cannot choose where on this machine the download lands.
+    /// </summary>
+    /// <remarks>
+    /// Backslashes and colons are ordinary in a POSIX file name and are separators and drive letters on
+    /// Windows, and a slash arrives in a name from an S3 key — so every one of these would have escaped the
+    /// folder through <c>Path.Combine</c> somewhere. They are queued inside it under a cleaned name, and the
+    /// bare <c>..</c>, which no cleaning can make a file name, is left on the host and said so.
+    /// </remarks>
+    [Fact]
+    public void ADownloadNamedToEscapeTheFolder_LandsInsideItOrNotAtAll()
+    {
+        Connected();
+
+        transfers.QueueDownloads(
+        [
+            RemoteFile(@"..\..\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\x.bat"),
+            RemoteFile(@"C:\Windows\evil.dll"),
+            RemoteFile("../../escaped.txt"),
+            RemoteFile(".."),
+        ]);
+
+        var landed = Queued().Select(row => row.Transfer.LocalPath).ToList();
+
+        landed.Count.ShouldBe(3);
+        landed.ShouldAllBe(path => Path.GetDirectoryName(path) == directory);
+        transfers.Status.ShouldContain("1 name points outside the folder");
+    }
+
     [Fact]
     public void TheDownloadButton_GoesThroughTheSamePathAsADrop()
     {