MainViewModel is now a partial class — scaffold for the per-area split (Commit 0)

Zero behavior change: \`public class\` -> \`public partial class\`. Each functional area
(Scenes, Background, Webcam, Audio, Trax, Socials, Streaming, Chat, Overlays,
Account, License) will move to its own file next, one commit per partial — each
a roll-back point. Docs updated in the same commit: ViewModels/index.md (split
map), ai.md (architecture table), HANDOFF.md (state). verify.sh: build 0 warnings,
only the 2 known pre-existing test failures.

HANDOFF.md

@@ -6,7 +6,7 @@
 
 No feature branches pre-1.0: all work lands on `main` per work unit.
 
-**Latest commit:** `6826786` — docs: add Distribution.md (IP protection plan with Polar.sh backbone).
+**Latest commit:** `6031a56` — gear cleanup + Default Location for Recordings + single Start + socials hover glow.
 
 ## What's In Flight
 
@@ -35,7 +35,7 @@ Nothing — clean tree, ready for next task.
 - TASK 22 (Audio sync offset): not started — required for v1.
 - TASK 30: gear cleanup + record location + single Start + preview layout restructure — shipped across commits `6031a56` + `61b850f` + `455a409`.
 - Monetization: deferred (creator rethinking price/subscription model).
-- **File refactor (next):** MainViewModel.cs (3,900 lines) will be split into functional partials (Scenes, Background, Webcam, Audio, Trax, Socials, Streaming, Chat, Overlays, Account, License). MainWindow.xaml (1,681 lines) will be split into UserControls. Zero behavior change, tests stay green. Decided this session.
+- **File refactor (in progress — Commit 0 done):** MainViewModel.cs (3,900 lines) will be split into functional partials (Scenes, Background, Webcam, Audio, Trax, Socials, Streaming, Chat, Overlays, Account, License). `MainViewModel` is now `public partial class` (scaffold commit). Remaining: one commit per partial, each a roll-back point, `verify.sh` + `scope-check.sh` before each, docs in the same commit. MainWindow.xaml (1,681 lines) will be split into UserControls after Phase 1. Zero behavior change, tests stay green.
 
 ## Session summary (2026-08-29)
 

ViewModels/MainViewModel.cs

@@ -21,7 +21,7 @@ using ytLive.Services.Encoder;
 
 namespace ytLive.ViewModels;
 
-public class MainViewModel : ViewModelBase
+public partial class MainViewModel : ViewModelBase
 {
     private readonly YouTubeAuthService _youtubeAuth;
     private readonly YouTubeStreamService _youtubeStream;

ai.md

@@ -154,7 +154,7 @@ C# / WPF (.NET 8) following MVVM:
 | Path | Role |
 |------|------|
 | `Models/` | Plain data types — Scene, Source (incl. `ClipShape`, `IsMirrored`, `VideoImageSource`), QualityOption, StreamConfig, StreamHealth, YouTubeChannel, ChatMessage, **Socials (`SocialService` enum + `SocialEntry`/`SocialsConfig` + `SocialServiceIcons`) — the social bar** |
-| `ViewModels/` | MainViewModel — exposes collections + commands for the UI; GoLiveViewModel, ReuseImageViewModel, CameraPickerViewModel, **SocialsDialogViewModel** |
+| `ViewModels/` | MainViewModel — `public partial class`, one file per functional area (Scenes, Background, Webcam, Audio, Trax, Socials, Streaming, Chat, Overlays, Account, License — split in progress, see `ViewModels/index.md`); GoLiveViewModel, ReuseImageViewModel, CameraPickerViewModel, **SocialsDialogViewModel** |
 | `Services/` | YouTube OAuth2, stream/broadcast management, live chat polling, LayoutStore (SQLite), **SocialValidator (`ISocialValidator` seam + `HttpSocialValidator` default)**, **webcam: `VideoFrame` seam + `CameraDeviceInfo`/`ICameraEnumerator`/`ICameraFrameSource` interfaces + `MediaCaptureCameraEnumerator`/`MediaCaptureFrameSource` (WinRT) + `CameraManager`**, **screen capture: `IFullScreenDetector`/`Win32FullScreenDetector` + `IScreenCaptureSource`/`ScreenCaptureFrameSource` (WinRT GraphicsCapture) + `ScreenCaptureManager` + `ScreenCaptureSourceFactory` + `Direct3D11Helper`/`CaptureInterop` (COM bridges)**, **compositor: `SceneCompositor` + `CompositorOptions` + pure `StretchMath` + `StaticPixelCache` (see "Scene compositor")**, **audio: `IAudioSource` seam + `WasapiLoopbackAudioSource`/`WasapiMicAudioSource` (NAudio WASAPI) + `AudioMixer` + pure `AudioLevelMeter`/`WaveToFloat`/`VoiceFilterChain`/`LowShelfFilter`/`HighShelfFilter`/`NoiseGate`/`Compressor`/`AutoDucker`/`AudioRingBuffer`/`TinyResampler` + `MusicPlayer` + `IAudioPipeWriter`/`NamedPipeAudioWriter` (see "Live audio capture")**, **encoder: `IFfmpegEncoder`/`FfmpegEncoder` + `IEncoderProcess`/`FfmpegEncoderProcess` + `IFfmpegLocator`/`FfmpegLocator` + pure `FfmpegArgs`/`FfmpegProgressParser`/`FfmpegEncoderPicker` + the `FramePump` frame producer (see "Live encoder" + "Live frame pipeline")**, **notifications: `INotificationService` seam (`AppNotificationSeverity` Info/Success/Warning/Error) + `NotificationService` (Notification.Wpf toasts, see "Toast notifications")** |
 | `Helpers/` | ViewModelBase (INotifyPropertyChanged), RelayCommand, ImageCache, AppLog (file logger), FocusPreservingListBox, OAuthCredentials, **TokenStore (DPAPI session persistence)**, visibility converters |
 | `Themes/` | `Controls.xaml` — the single dark-theme source, merged once in `App.xaml` (see `Themes/index.md`) |