docs(plan): add set_window_opacity Rust command

Tauri 2 JS WebviewWindow does not expose setOpacity (only the Rust
side has set_opacity), so the JS layer routes through a custom
command. Surfaced during Task 9 implementation; updates Tasks 16 and
17 to register the command.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/superpowers/plans/2026-05-08-browserlay-mvp.md

@@ -1881,6 +1881,20 @@ pub fn get_hotkey_status(app: AppHandle) -> Result<crate::HotkeyStatus, String>
     let guard = state.hotkey_status.lock().map_err(|_| "poisoned".to_string())?;
     Ok(guard.clone())
 }
+
+/// Set window opacity. Tauri 2's JS WebviewWindow does NOT expose setOpacity
+/// (only the Rust side does), so the JS layer routes through this command.
+#[tauri::command]
+pub fn set_window_opacity<R: Runtime>(
+    app: AppHandle<R>,
+    label: String,
+    opacity: f32,
+) -> Result<(), String> {
+    let Some(window) = app.get_webview_window(&label) else {
+        return Err(format!("window not found: {label}"));
+    };
+    window.set_opacity(opacity).map_err(|e| e.to_string())
+}
 ```
 
 > **Note on the design:** Tauri 2 doesn't expose a getter for `ignore_cursor_events` / `opacity` on `WebviewWindow`. Rather than rely on getters, we keep a tiny Rust-side cache (`AppState.click_through`) that the JS side keeps in sync via `sync_click_through_cache`. The global shortcut handler reads this cache, computes the inverse, calls `apply_state`. The JS-driven toggle path uses `toggle_click_through(current)` and skips the cache.
@@ -1994,6 +2008,7 @@ pub fn run() {
             commands::overlay::toggle_click_through,
             commands::overlay::sync_click_through_cache,
             commands::overlay::get_hotkey_status,
+            commands::overlay::set_window_opacity,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");