Resolution dropdown, list-focus preservation, unified dark theme, startup logging, and memory map

- Bottom-bar resolution dropdown (1080p60/1080p30/720p60/480p30) with tooltip
  on finding upload bandwidth; disabled while live; no in-app speed test
- FocusPreservingListBox keeps selection/focus coherent when a selected
  scene/source is deleted (skip hidden scenes; leave list when empty)
- Themes/Controls.xaml: single dark-theme dictionary merged once in App.xaml;
  custom ComboBox template fixes SelectionBoxItem rendering; GoLiveWindow and
  ReuseImageDialog consolidated onto shared styles
- AppLog file logger + AppDomain/Dispatcher exception hooks; checkpointed
  MainWindow/VM/dialog constructors (caught MenuItemRole.Separator XAML crash)
- Memory map: schema.md conventions, per-directory index.md, updated ai.md/
  TASKS.md/README.md (OAuth creds real; token persistence still pending)

Helpers/AppLog.cs

@@ -0,0 +1,40 @@
+using System.IO;
+
+namespace ytLive.Helpers;
+
+/// <summary>
+/// Minimal file logger used to diagnose startup/shutdown crashes on Windows,
+/// where the WPF app has no visible console. Writes to
+/// %APPDATA%\ytLlive\startup.log, appending synchronously so entries survive
+/// a hard crash.
+/// </summary>
+public static class AppLog
+{
+    private static readonly string LogPath = Path.Combine(
+        Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+        "ytLlive",
+        "startup.log");
+
+    private static readonly object Gate = new();
+
+    public static void Write(string message)
+    {
+        try
+        {
+            lock (Gate)
+            {
+                Directory.CreateDirectory(Path.GetDirectoryName(LogPath)!);
+                File.AppendAllText(LogPath, $"{DateTime.Now:yyyy-MM-dd HH:mm:ss.fff} {message}{Environment.NewLine}");
+            }
+        }
+        catch
+        {
+            // Never let logging itself crash the app.
+        }
+    }
+
+    public static void Write(Exception exception, string message)
+    {
+        Write($"{message}: {exception}");
+    }
+}