refactor: extract OverlayHost (settings/bug/feature/about dialogs) — Phase 2, Commit 18

OverlayHost.xaml(.cs) takes the window-root overlay dialog grid (settings/
bug/feature/about panels); BackToAbout_Click moved in via inherited DataContext.
WebViewHostPanel + ToastArea stay as thin window-root stubs so the WebView2 ctor
init and NotificationService InArea('ToastArea') name lookups are untouched.
This completes Phase 2: MainWindow.xaml is now a 123-line shell.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 2 tracker updated in same commit.

ViewModels/index.md

@@ -60,6 +60,7 @@ Progress (Commit N/12 · tag):
 - [x] **Commit 15 · `refactor-commit-15`** — **PreviewPane** → `Controls/PreviewPane.xaml(.cs)` (Row 1 Col 1 center preview pane). Moved the 1920×1080 canvas (OverlayCanvas/SelectionOverlay/BrandFlashLayer/SocialBarElement), text pull-out, and the live-controls row (TRAX/game/mic meters + sliders + speakers + socials button). Code-behind moved: preview drag/select/resize/hit-test (Preview_Mouse*, HitHandle, HitElement), webcam-context-menu handlers (WebcamMenu_*), social-bar glow (SocialBar_*, SocialsButton_*, SocialGlowTick), audio sliders/speakers (VolumeSlider_*, GameVolumeSlider_*, MicSpeaker_*, GameSpeaker_*, SetSliderValueFromClick, FindParent), TraxButton_*. Window keeps the source-list left panel + thumbnail strip + edit UI, and routes selection-overlay via `_previewPane.UpdateSelectionOverlay(bool)` plus `_previewPane.IsClickInsidePreview/Drawer`. `BackgroundMenu_*` stayed in the window (used by the left-panel source context menu).
 - [x] **Commit 16 · `refactor-commit-16`** — **SceneThumbnailStrip** → `Controls/SceneThumbnailStrip.xaml(.cs)` (Row 0 scene-picker strip). `Thumbnail_MouseLeftButtonDown` moved into the control (transitions via `DataContext.TransitionToScene`).
 - [x] **Commit 17 · `refactor-commit-17`** — **LeftPanel** → `Controls/LeftPanel.xaml(.cs)` (Row 1 Col 0 left panel: layers/properties edit UI + live chat list). Moved ALL property-editor and chat handlers: `SourceNameBox_*`/`CommitSourceEdit`, `AddSourceButton_Click`, `OnChatMessagesChanged` (now self-subscribed on Loaded), `MirrorButton_Click`, `ShapeButton_Click`, `SourceMenu_Remove`, `BackgroundMenu_*`, `OpacitySlider_ValueChanged`, list drag-to-reorder (`_dragStartPoint`+`List_Preview*`+`EndListDrag`+`FindItemContainerAt`), and the snapshot family (`TakeSnapshot`, `WebcamBorder*`, `Countdown*`, `Web*`). Window exposes via `_leftPanel.IsClickInside(original)` (deselect guard) and `_leftPanel.OnSelectionChanged(vm)` (opacity label + snapshot). Pending: overlay host region.
+- [x] **Commit 18 · `refactor-commit-18`** — **OverlayHost** → `Controls/OverlayHost.xaml(.cs)` (settings/bug/feature/about dialogs). `BackToAbout_Click` moved with it (uses inherited `DataContext`). Window keeps the thin `WebViewHostPanel` + `ToastArea` root stubs so name lookups are untouched (WebView2 ctor init, `NotificationService.InArea("ToastArea")`) — toast/WebView2 behavior unchanged. **Phase 2 COMPLETE** — MainWindow.xaml carved down to a 123-line shell over 6 controls (TopBar, HealthBanner, BottomBar, SceneThumbnailStrip, LeftPanel, PreviewPane, OverlayHost). Pending: session wrap-up (HANDOFF.md) + push on approval.
 
 The partial `.cs` files land next to `MainViewModel.cs` in this folder as the split
 proceeds; each partial carries its own `using`s and re-declares nothing from core.