refactor: extract LeftPanel out of MainWindow.xaml — Phase 2, Commit 17

LeftPanel.xaml(.cs) takes the Row 1 Col 0 panel (layers + properties editor and
the live chat list). All property-editor/chat handlers moved into the control
(SourceNameBox edit, AddSourceButton menu, chat autoscroll self-subscribed on
Loaded, mirror/clip-shape, remove-source, background capture change/refresh,
opacity slider label, list drag-to-reorder, snapshot revert/accept family).

Window keeps only shell + routing: ctor wiring (_leftPanel), geometry, hotkeys,
taskbar overlay, BackToAbout, Window_PreviewMouseLeftButtonDown (uses
_leftPanel.IsClickInside), UpdateSelectionOverlay (calls _leftPanel.OnSelectionChanged).

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

@@ -58,7 +58,8 @@ Progress (Commit N/12 · tag):
 - [x] **Commit 13 · `refactor-commit-13`** — **HealthBanner** → `Controls/HealthBanner.xaml` (Row 1). Also promoted the 7 value↔visibility converters out of `MainWindow.Window.Resources` into `Themes/Controls.xaml` so extracted controls can resolve them (app-level, not window-level).
 - [x] **Commit 14 · `refactor-commit-14`** — **TopBar** → `Controls/TopBar.xaml(.cs)` (Row 0: logo, REC/ON-AIR pills + signs, avatar, Sign In/Start/End buttons). Avatar fetch moved (async-preserved) into the control; window exposes `_topBar.RefreshAvatar()` invoked on `IsConnected`/`AccountAvatarUrl` changes.
 - [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`). Pending: left scene/chat panel, overlay host.
+- [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.
 
 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.