Give hosts and terminals their own screen, and the rest of the vault another

Rebuilds the client's shell from an imported design: a titlebar and nav rail
it draws itself, real multi-session tabs over the one WebView, a Ctrl+K host
search, and a vault screen that merges keys, passwords and pinned host keys
into one table. Hosts left the vault column for their own screen beside the
terminal, which is what the design asks for and turned out to be the better
split anyway.

Two screens the design shows have nothing behind them yet — file transfer
and teams — and say so plainly rather than rendering invented data; every
other gap between the design and this build is recorded in
docs/design-import-gaps.md.

tests/DodoSSH.Client.App.Layout.Tests/LayoutHarnessTests.cs

@@ -140,9 +140,9 @@ public sealed class LayoutHarnessTests
         // A HeadlessUnitTestSession owns its dispatcher thread and does not offer an apartment choice, so
         // the whole window cannot be laid out here at any size.
         //
-        // That is the reason this harness measures VaultColumn rather than MainWindow: the column is the
-        // part with a height budget to blow, and it has no native child window in it. If a future Avalonia
-        // makes the adapter lazy, this test starts failing and the harness can be widened.
+        // That is the reason this harness measures the extracted controls rather than MainWindow: each of
+        // them is a part with a height budget to blow, and none of them has a native child window in it. If a
+        // future Avalonia makes the adapter lazy, this test starts failing and the harness can be widened.
         await LayoutHarness.OnTheUiThreadAsync(
             () =>
             {
@@ -170,7 +170,7 @@ public sealed class LayoutHarnessTests
     [Fact]
     public async Task TheHarnessMeasuresTheSizeTheWindowDeclares()
     {
-        // Pins the two constants against the XAML. A harness measuring 820x520 while the window lets itself
+        // Pins the two constants against the XAML. A harness measuring a size the window lets itself
         // be dragged to something smaller would be certifying a size no user is held to.
         await LayoutHarness.OnTheUiThreadAsync(
             () =>