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/index.md

@@ -0,0 +1,17 @@
+# Helpers — index
+
+Cross-cutting utilities. See [`schema.md`](../schema.md) for the memory-map conventions.
+
+| File | Purpose |
+|------|---------|
+| `ViewModelBase.cs` | `INotifyPropertyChanged` + `SetProperty<T>()` — base for all ViewModels |
+| `RelayCommand.cs` | `ICommand` implementation for all button actions |
+| `AppLog.cs` | File logger to `%APPDATA%\ytLlive\startup.log`; startup checkpoints + unhandled-exception capture (the crash this map is named for) |
+| `FocusPreservingListBox.cs` | `ListBox` subclass that keeps selection/focus coherent when the selected item is deleted; `IsSelectable` predicate skips hidden scenes; drag-reorder guard |
+| `OAuthCredentials.cs` | Baked-in Google OAuth client ID/secret (desktop app; loopback callback) |
+| `ImageCache.cs` | Image byte caching (assets live in the DB) |
+| `InverseBoolToVisibilityConverter.cs` / `NotNullToVisibilityConverter.cs` | XAML value converters for visibility bindings |
+
+Related: [`Themes/Controls.xaml`](../Themes/Controls.xaml) styles the lists this
+class backs; [`Models/index.md`](../Models/index.md) and
+[`ViewModels/index.md`](../ViewModels/index.md) for the objects it works on.