Add white/black Folio icon variants and theme-aware in-app icon

- Recolor the in-app glyph (Design B) to a solid white master and generate a
  black variant; the desktop/app icon set (build/icons) is now the white
  master at all sizes (verified bundled white in the AppImage/deb)
- In-app icon switches with the existing data-theme: black in Light mode
  (visible on the light panel) and white in Dark mode (visible on the
  near-black panel); the desktop/application icon stays white

esbuild.config.mjs

@@ -30,6 +30,10 @@ const copyAssets = {
         path.join("build", "icons", "128x128.png"),
         path.join("dist", "brand-icon.png")
       );
+      fs.copyFileSync(
+        path.join("build", "brand-icons", "128x128-black.png"),
+        path.join("dist", "brand-icon-black.png")
+      );
     });
   },
 };