design: every window inherits Inter (implicit Window style)

Dialogs are separate WPF windows, so the FontFamily set on MainWindow never reached them —
GoLive, Socials and the four pickers still rendered Segoe UI while the main window used Inter.

The theme now carries an implicit Window style (family + Display text mode + layout rounding).
Guard: DialogTypographyTests opens a real Window inside the real app and asserts it resolves the
bundled Inter family — which fails if the style or the font resource goes missing — and asserts
no dialog sets FontFamily itself, i.e. nobody opts out of the standard.

Themes/Controls.xaml

@@ -2,6 +2,16 @@
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                     xmlns:Helpers="clr-namespace:ytLive.Helpers">
 
+    <!-- Typography for every window: dialogs are separate WPF windows, so FontFamily set on
+         MainWindow does not reach them. This implicit style is the one place that does — a
+         dialog inherits Inter without setting anything, and the design standard holds for every
+         surface the creator can open. -->
+    <Style TargetType="Window">
+        <Setter Property="FontFamily" Value="{StaticResource FontFamily.Sans}"/>
+        <Setter Property="TextOptions.TextFormattingMode" Value="Display"/>
+        <Setter Property="UseLayoutRounding" Value="True"/>
+    </Style>
+
     <!-- Base button style -->
     <Style x:Key="YtButton" TargetType="Button">
         <Setter Property="Background" Value="{StaticResource Brush.Tertiary}"/>

ytLive.Tests/DialogTypographyTests.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Windows;
+using Xunit;
+
+namespace ytLive.Tests;
+
+/// <summary>DESIGN.md names Inter for all UI text, but a dialog is a SEPARATE WPF window: the
+/// FontFamily set on MainWindow never reaches it, so every dialog still rendered Segoe UI while
+/// the main window had moved on (found 2026-10-30).
+///
+/// <para>The theme now carries an implicit <c>Window</c> style. This test proves the mechanism on
+/// a real window inside the real app — which is what catches a missing resource or a broken
+/// implicit-style lookup — and asserts no dialog opts out by setting FontFamily itself.</para></summary>
+[Collection("RealApp")]
+public class DialogTypographyTests
+{
+    private readonly RealAppHost _app;
+
+    public DialogTypographyTests(RealAppHost app) => _app = app;
+
+    [Fact]
+    public void A_window_inherits_the_standard_typeface_and_no_dialog_opts_out()
+    {
+        // 1. The mechanism: any Window opened in the app resolves the token family. Dialogs are
+        //    Windows; proving it once here covers all of them, and it fails if the implicit style
+        //    or the font resource ever goes missing.
+        _app.Run(() =>
+        {
+            var window = new Window();
+            window.Show();
+            try
+            {
+                Assert.Contains("Inter", window.FontFamily?.Source ?? string.Empty,
+                    StringComparison.OrdinalIgnoreCase);
+            }
+            finally { window.Close(); }
+        });
+
+        // 2. Opt-outs: a dialog that sets FontFamily itself would silently leave the standard.
+        foreach (var file in DialogFiles())
+            Assert.DoesNotContain("FontFamily=", File.ReadAllText(file));
+    }
+
+    private static IEnumerable<string> DialogFiles()
+    {
+        var root = Path.GetDirectoryName(SourceAssert.RepoPath("App.xaml"))!;
+        return Directory.EnumerateFiles(root, "*.xaml", SearchOption.AllDirectories)
+            .Where(p => !p.Contains($"{Path.DirectorySeparatorChar}obj{Path.DirectorySeparatorChar}", StringComparison.Ordinal)
+                     && !p.Contains($"{Path.DirectorySeparatorChar}bin{Path.DirectorySeparatorChar}", StringComparison.Ordinal)
+                     && !p.Contains("ytLive.Tests", StringComparison.Ordinal))
+            .Where(p => Path.GetFileName(p).Contains("Dialog", StringComparison.OrdinalIgnoreCase)
+                     || Path.GetFileName(p).StartsWith("GoLiveWindow", StringComparison.OrdinalIgnoreCase));
+    }
+}