refactor: eliminate startup flicker — defer window show until overlay renders

- Remove Rust-side `eval(INITIAL_LOADING_OVERLAY)` that prematurely
  dismissed the overlay before React/MUI theme was ready
- Defer `window.show()` from Rust `activate_window` to an inline
  `<script>` in index.html, executed after the themed overlay is in DOM
- Remove `useAppInitialization` hook (duplicate of `useLoadingOverlay`
  with no themeReady gate)
- Simplify overlay to pure theme-colored background — no spinner or
  loading text — so fast startup feels instant
- Simplify `hideInitialOverlay` API and reduce overlay fade to 0.2s
- Clean up unused CSS variables (spinner-track, spinner-top, etc.)

src-tauri/src/utils/window_manager.rs

@@ -285,6 +285,7 @@ impl WindowManager {
     }
 
     /// 创建新窗口,防抖避免重复调用
+    /// 窗口创建后保持隐藏，由前端 index.html 在 overlay 渲染后调用 show，避免主题闪烁
     pub fn create_window(is_show: bool) -> Pin<Box<dyn Future<Output = bool> + Send>> {
         Box::pin(async move {
             logging!(info, Type::Window, "开始创建/显示主窗口, is_show={}", is_show);
@@ -293,23 +294,22 @@ impl WindowManager {
                 return false;
             }
 
-            let window = match build_new_window().await {
-                Ok(window) => {
-                    logging!(info, Type::Window, "新窗口创建成功");
-                    window
+            match build_new_window().await {
+                Ok(_) => {
+                    logging!(info, Type::Window, "新窗口创建成功，等待前端渲染后显示");
+
+                    #[cfg(target_os = "macos")]
+                    {
+                        handle::Handle::global().set_activation_policy_regular();
+                    }
+
+                    true
                 }
                 Err(e) => {
                     logging!(error, Type::Window, "新窗口创建失败: {}", e);
-                    return false;
+                    false
                 }
-            };
-
-            // 直接激活刚创建的窗口，避免因防抖导致首次显示被跳过
-            if WindowOperationResult::Failed == Self::activate_window(&window) {
-                return false;
             }
-
-            true
         })
     }