fix: creator feedback batch Branch 1 — TRAX volume, tooltip, meter boost, slider click, backdrop

- TRAX: LocalGain now applies 0.2× factor so music is quieter relative to desktop audio slider
- TRAX tooltip: switched to ToolTipService with Placement=Top to fix z-order in Viewbox overlay
- Mic meter: MeterLevel multiplied by 1.2× before clamping for ~20% higher visual readout
- Volume sliders: click anywhere on track to jump thumb to that position before dragging
- Backdrop: Image Visibility now binds to BackdropVisible; compositor checks IsVisible before blit
- Renamed 'Backdrop' to 'Game Capture' in EnsureBackdrop + test assertions

Services/Compositor/SceneCompositor.cs

@@ -46,7 +46,7 @@ public sealed class SceneCompositor
         var elements = scene.Elements;
 
         var backdrop = elements.OfType<Source>().FirstOrDefault(s => s.IsBackdrop);
-        var backdropFrame = backdrop != null ? frameFor(backdrop) : null;
+        var backdropFrame = backdrop != null && backdrop.IsVisible ? frameFor(backdrop) : null;
         if (backdropFrame != null)
             BlitContent(buffer, cropW, cropH, 0, 0, cropW, cropH, backdropFrame, 1f, false, false);