Stop the terminal's WebView painting over the setup screens

The shell layered its setup and unlock screens over the terminal, which does
not work: NativeWebView attaches a real Win32 child HWND through
NativeControlHost, and a child window composites above everything its parent
paints regardless of visual-tree z-order. The cards rendered sliced at the
terminal column's left edge; at the window's default width every one of their
buttons fell inside the WebView's rectangle, so the flow could only be
completed by keyboard, and a click in that region handed Win32 focus to
WebView2 so the text boxes silently stopped accepting keystrokes.

The WebView is now collapsed while the vault is not unlocked. The comment
that previously forbade this — hiding it means never realising it — was
wrong: NativeControlHost creates the native attachment on attach to the
visual tree, never consulting layout or visibility, and NativeWebView replays
a Source assigned before its adapter exists. A collapsed WebView still starts
WebView2, loads the page and lets the renderer attach. Confirmed: 35
msedgewebview2 processes with the control collapsed. What the first
connection after unlocking actually depends on is the existing await on
WaitForRendererAsync, since the data plane drops frames when no renderer is
attached.

Also fixes the second visible defect: the default server URL was
https://localhost:7217, the API's *second* launch profile, while the README,
its appsettings and a plain `dotnet run` all use http://localhost:5233 — so
nothing was listening, and an HTTPS client against a plaintext port reports
"The SSL connection could not be established", which reads as a certificate
problem. The default now matches, a missing scheme is rejected by name
instead of parsing as scheme "localhost", and that specific TLS failure now
suggests http://. Both new tests fail when the fixes are reverted.

Corrections to claims I made earlier and should not have:

- docs/platform-flags.md asserted the opposite of the mechanism above and
  cited an established msedgewebview2 connection as verification. That
  observation was taken while the overlay was showing but, because of this
  very bug, the WebView was uncovered and in plain view — so it confirmed
  only that a visible WebView is realised. A process-level check cannot
  verify a rendering claim. The entry was also filed under "Local cache".
- ITerminalHost was documented as the live seam the app plugs into, with a
  stub standing in for headless tests. It has no implementation anywhere and
  no test uses it; the view navigates the control directly. It also counted
  Avalonia.Controls.WebView and NativeWebView as two interchangeable
  backends when they are one component, with the Linux backend backwards.
- The README claimed the shell's whole path was covered by tests. Its state
  machine is; its layout is covered by nothing, and a headless test could
  not have caught this — headless has no native window, so it would have
  rendered correctly and confirmed the wrong belief.

Verified by screenshotting the running app: the card renders complete and
centred at the default size, with the button clickable.

src/DodoSSH.Client.App/ViewModels/MainWindowViewModel.cs

@@ -1,3 +1,4 @@
+using System.Security.Authentication;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using DodoSSH.Client.Auth;
@@ -105,8 +106,18 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
     [ObservableProperty]
     private bool isBusy;
 
+    /// <remarks>
+    /// The address <c>dotnet run --project src/DodoSSH.Api</c> actually serves, so the first launch after
+    /// a clone works without the user having to know a port. This was <c>https://localhost:7217</c>, which
+    /// is the API's <em>second</em> launch profile: the first is HTTP on 5233 and is the one both the
+    /// README and a plain <c>dotnet run</c> select, so nothing was listening on 7217. Pointing an HTTPS
+    /// client at a plaintext port fails as "The SSL connection could not be established", which sends
+    /// people looking for a certificate problem — see <see cref="ExplainSignInFailure" />. A real
+    /// deployment is HTTPS behind a proxy and its address is typed over this one; the placeholder in the
+    /// setup card shows that shape.
+    /// </remarks>
     [ObservableProperty]
-    private string serverUrl = "https://localhost:7217";
+    private string serverUrl = "http://localhost:5233";
 
     [ObservableProperty]
     private string passphrase = string.Empty;
@@ -192,9 +203,19 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
             return;
         }
 
+        // Checked separately from parsing, because "localhost:5233" parses perfectly well as an absolute
+        // URI whose scheme is "localhost" — and then fails much later with something unrelated to the
+        // actual mistake.
+        if (url.Scheme is not ("http" or "https"))
+        {
+            StatusMessage = $"A server URL has to start with http:// or https://, not {url.Scheme}:.";
+            return;
+        }
+
         await RunAsync(
             "Opening your browser to sign in…",
-            async () =>
+            explain: exception => ExplainSignInFailure(exception, url),
+            work: async () =>
             {
                 connection?.Dispose();
                 connection = null;
@@ -383,7 +404,17 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
     /// Every command funnels through here so the busy flag and the failure message are handled once. A
     /// command that forgot either would leave the window permanently disabled or silently doing nothing.
     /// </remarks>
-    private async Task RunAsync(string busyMessage, Func<Task> work)
+    /// <param name="busyMessage">Shown while the work runs.</param>
+    /// <param name="work">The work.</param>
+    /// <param name="explain">
+    /// Turns a failure into something a user can act on. Optional, because most failures here already
+    /// carry their own explanation; the ones that do not are the ones crossing into another process's
+    /// vocabulary, where the exception describes a symptom and not the mistake.
+    /// </param>
+    private async Task RunAsync(
+        string busyMessage,
+        Func<Task> work,
+        Func<Exception, string>? explain = null)
     {
         if (IsBusy)
         {
@@ -403,7 +434,7 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
         }
         catch (Exception exception)
         {
-            StatusMessage = exception.Message;
+            StatusMessage = explain?.Invoke(exception) ?? exception.Message;
         }
         finally
         {
@@ -411,6 +442,28 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
         }
     }
 
+    /// <remarks>
+    /// One case earns a translation rather than the exception's own words: pointing an HTTPS client at a
+    /// plaintext port reports "The SSL connection could not be established", which sends people looking
+    /// for a certificate problem. The scheme is the mistake, and the development stack serves HTTP, so
+    /// this is the first thing a new user will hit.
+    /// </remarks>
+    private static string ExplainSignInFailure(Exception exception, Uri server)
+    {
+        var secureChannelFailed = exception is HttpRequestException
+            && exception.GetBaseException() is AuthenticationException;
+
+        if (secureChannelFailed && server.Scheme is "https")
+        {
+            var plain = new UriBuilder(server) { Scheme = "http" }.Uri;
+
+            return $"{exception.Message} {server.Host} answered, but not with TLS. If this is a "
+                + $"development server it probably serves plain HTTP — try {plain.GetLeftPart(UriPartial.Authority)}.";
+        }
+
+        return exception.Message;
+    }
+
     partial void OnStateChanged(ShellState value)
     {
         OnPropertyChanged(nameof(IsStarting));