Add the Avalonia app and the xterm renderer, and fix two real bugs

The terminal works end to end. A new integration test drives a real sshd in
a container through a real PTY, the real pump, the real loopback WebSocket
with its token and origin checks, and a ClientWebSocket standing in for the
page: the login banner arrives, typed input round-trips, and `stty size`
reports the 100x30 the session asked for. The only untested link left is
xterm drawing bytes it was handed.

The WebView is de-risked on Windows, which was the plan's largest risk. Not
by assertion: with the app running there is an established TCP connection
from msedgewebview2 to the data plane port, so WebView2 launched, navigated
to the loopback page, executed terminal.js, and completed the WebSocket
handshake against the real token and origin checks. Linux remains unproven
and the package's own release notes now corroborate the concern -- Linux uses
a WPE backend, and it ships a NativeWebDialog described as useful where
embedded WebViews may be unavailable.

Two bugs found by building it, both of which would have shipped:

- ShellStream.Write buffers and needs an explicit Flush. Without one a
  keystroke is accepted, reported as written, and never reaches the remote:
  the terminal displays output perfectly and simply stops responding to
  input. SSH.NET's own WriteLine flushes, which is why the earlier spike
  never hit it. Found by isolating the pump against real SSH and reading
  BytesRead=51 -- banner and prompt through, nothing after.
- The Windows app manifest needs a supportedOS list, or Avalonia's native
  control host fails outright and the terminal never starts.

Also fixed a genuinely flaky test I happened to catch: SyncCursorTests
tampered with the *last* base64url character, whose low bits the decoder
ignores when the input length is not a multiple of three -- so a tampered
cursor sometimes decoded to identical bytes and verified. It failed roughly
one run in thirty, depending on a random key. Now tampers the penultimate
character, which is fully significant at every length; 40 consecutive runs
are clean.

xterm 6.0.0 plus the fit and webgl addons are vendored as UMD bundles rather
than built with npm, so a clean clone needs only the .NET SDK. Provenance
and licences are recorded next to them, along with the UMD global names
terminal.js depends on -- a bundle that switched to ES modules would load
without error and leave Terminal undefined.

The renderer acknowledges output from term.write's completion callback, not
on receipt. Acknowledging early would return flow-control credit for bytes
the screen has not caught up with, which is the one thing the credit window
exists to measure.

TerminalWorkspace moved into DodoSSH.Client.Terminal: it has no Avalonia
dependency, and having it there is what let the end-to-end test exist at all.

404 tests pass, zero warnings on a clean rebuild, format clean.

src/DodoSSH.Client.App/Views/MainWindow.axaml

@@ -0,0 +1,75 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:vm="using:DodoSSH.Client.App.ViewModels"
+        x:Class="DodoSSH.Client.App.Views.MainWindow"
+        x:DataType="vm:MainWindowViewModel"
+        Title="DodoSSH"
+        Width="1100"
+        Height="720"
+        MinWidth="640"
+        MinHeight="400"
+        Background="#10131a">
+
+  <Grid RowDefinitions="Auto,Auto,*">
+
+    <!-- Connection bar. Replaced by the host list once the vault is wired up. -->
+    <Border Grid.Row="0" Padding="10,8" Background="#171b24">
+      <StackPanel Orientation="Horizontal" Spacing="8">
+        <TextBox Text="{Binding Host}" PlaceholderText="host" Width="200" VerticalAlignment="Center" />
+        <NumericUpDown Value="{Binding Port}" Minimum="1" Maximum="65535"
+                       FormatString="0" Width="110" VerticalAlignment="Center" />
+        <TextBox Text="{Binding Username}" PlaceholderText="username" Width="150" VerticalAlignment="Center" />
+        <TextBox Text="{Binding Password}" PlaceholderText="password" PasswordChar="•"
+                 Width="170" VerticalAlignment="Center" />
+        <Button Content="Connect"
+                Command="{Binding ConnectCommand}"
+                IsEnabled="{Binding !IsConnecting}"
+                VerticalAlignment="Center" />
+        <TextBlock Text="{Binding Status}" Foreground="#7b8394"
+                   VerticalAlignment="Center" TextTrimming="CharacterEllipsis" />
+      </StackPanel>
+    </Border>
+
+    <!--
+      Host key prompts. Unknown and changed look deliberately different: one is a decision, the
+      other is a refusal. Presenting a changed key with a "continue" button is how users are taught
+      to click through the one warning that matters.
+    -->
+    <StackPanel Grid.Row="1">
+
+      <Border Padding="10,8" Background="#2b2410" IsVisible="{Binding HasPendingHostKey}">
+        <StackPanel Spacing="6">
+          <TextBlock Text="This host has not been seen before. Check the fingerprint against what the server's operator published."
+                     Foreground="#e8dcb0" TextWrapping="Wrap" />
+          <SelectableTextBlock Text="{Binding PendingHostKey.Fingerprint}"
+                               FontFamily="ui-monospace,Consolas,monospace"
+                               Foreground="#f4ecd0" />
+          <StackPanel Orientation="Horizontal" Spacing="8">
+            <Button Content="Trust and connect" Command="{Binding TrustHostKeyCommand}" />
+            <Button Content="Cancel" Command="{Binding RejectHostKeyCommand}" />
+          </StackPanel>
+        </StackPanel>
+      </Border>
+
+      <Border Padding="10,8" Background="#3a1418" IsVisible="{Binding HasHostKeyMismatch}">
+        <StackPanel Spacing="6">
+          <TextBlock Text="The host key changed and the connection was refused."
+                     Foreground="#f3c9cd" FontWeight="SemiBold" />
+          <SelectableTextBlock Text="{Binding HostKeyMismatch}"
+                               Foreground="#f3c9cd" TextWrapping="Wrap" />
+          <TextBlock Text="If the server was legitimately rebuilt, remove its pinned key in the host's settings first. There is deliberately no way to continue from here."
+                     Foreground="#d59aa1" TextWrapping="Wrap" />
+        </StackPanel>
+      </Border>
+
+    </StackPanel>
+
+    <!--
+      One WebView hosting every terminal. Not one per tab: each WebView2 is a separate browser
+      process, so twenty tabs would cost twenty renderer processes.
+    -->
+    <NativeWebView Grid.Row="2" x:Name="Terminal" />
+
+  </Grid>
+
+</Window>