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.

src/DodoSSH.Client.App/Views/PreferencesScreen.axaml.cs

@@ -0,0 +1,9 @@
+using Avalonia.Controls;
+
+namespace DodoSSH.Client.App.Views;
+
+/// <summary>Preferences: what this build can actually change, and a list of what it cannot.</summary>
+internal sealed partial class PreferencesScreen : UserControl
+{
+    public PreferencesScreen() => InitializeComponent();
+}