fix(dictation): mirror the widget's shadow setting in the macOS overlay

Tauri applies platform config as a JSON Merge Patch, so the windows array in
tauri.macos.conf.json REPLACES the base one rather than merging window objects.
desktopWindowConfig.test.js pins that every base window property survives on
macOS, and it caught the base gaining "shadow": false without the overlay.

The programmatic builder already sets shadow(false) on every platform; this
keeps the macOS declaration in step with it. The Linux and Windows overlays
declare no widget window, so the base entry applies there unchanged.

frontend/src-tauri/tauri.macos.conf.json

@@ -27,6 +27,7 @@
         "fullscreen": false,
         "transparent": true,
         "decorations": false,
+        "shadow": false,
         "alwaysOnTop": true,
         "visible": false,
         "skipTaskbar": true,