refactor: extract BottomBar out of MainWindow.xaml — Phase 2, Commit 12

New Controls/ folder + namespace ytLive.Controls. BottomBar takes the
Grid.Row=3 footer: gear menu button (with its GearButton_Click handler moved to
the control's code-behind), centered stream-stats (bitrate/FPS/dropped/
duration/health), and the quality dropdown. All bindings inherit the window
DataContext; app-level styles resolve from Themes/Controls.xaml as before.

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

ViewModels/index.md

@@ -51,7 +51,10 @@ Progress (Commit N/12 · tag):
 - [x] **Commit 9/12 · `refactor-commit-9`** — **Streaming** → `MainViewModel.Streaming.cs`
 - [x] **Commit 10/12 · `refactor-commit-10`** — **Chat** → `MainViewModel.Chat.cs`
 - [x] **Commit 11/12 · `refactor-commit-11`** — **Overlays** → `MainViewModel.Overlays.cs`
-- [x] **Phase 1 complete:** core `MainViewModel.cs` split into 10 functional partials (1,890 → 758 lines). Phase 2 (splitting `MainWindow.xaml` into UserControls) not yet started.
+- [x] **Phase 1 complete:** core `MainViewModel.cs` split into 10 functional partials (1,890 → 758 lines).
+
+**Phase 2 — `MainWindow.xaml` split into UserControls** (new `Controls/` folder, namespace `ytLive.Controls`):
+- [x] **Commit 12 · `refactor-commit-12`** — **BottomBar** → `Controls/BottomBar.xaml(.cs)` (gear menu + stream stats + quality dropdown; `GearButton_Click` moved to the control's code-behind). Pending: TopBar, health banner, left scene/chat panel, preview pane, overlay host.
 
 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.