workspace: Rework background image to VS Code-style surface tint

Replaces the whole-UI opacity hack with a theme-level surface tint:

- background.opacity now scales the alpha of editor/terminal/panel
  surface colors only, so the image shows through the code area while
  text, borders, tabs and chrome stay fully opaque and readable
- the image element renders at full opacity behind the workspace
- theme reloads automatically when background settings change

assets/settings/default.json

@@ -604,11 +604,10 @@
     "path": null,
     // How the image is fitted into the workspace bounds: "cover", "fill" or "contain".
     "fit": "cover",
-    // The opacity of the background image, between 0.0 (fully transparent) and 1.0 (fully opaque).
-    "opacity": 0.6,
-    // The opacity of the UI drawn on top of the image, between 0.0 (UI fully
-    // transparent, image fully visible) and 1.0 (opaque UI).
-    "ui_opacity": 1.0,
+    // The opacity of the editor and terminal surfaces drawn on top of the image,
+    // between 0.0 (surfaces fully transparent, image fully visible) and 1.0
+    // (surfaces fully opaque, image hidden).
+    "opacity": 0.15,
   },
   // Scrollbar related settings
   "scrollbar": {