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/resolve/window.rs

@@ -2,11 +2,7 @@ use dark_light::{Mode as SystemTheme, detect as detect_system_theme};
 use tauri::utils::config::Color;
 use tauri::{Theme, WebviewWindow};
 
-use crate::{
-    config::Config,
-    core::handle,
-    utils::resolve::window_script::{INITIAL_LOADING_OVERLAY, build_window_initial_script},
-};
+use crate::{config::Config, core::handle, utils::resolve::window_script::build_window_initial_script};
 use clash_verge_logging::{Type, logging_error};
 
 const DARK_BACKGROUND_COLOR: Color = Color(46, 48, 61, 255); // #2E303D
@@ -82,7 +78,6 @@ pub async fn build_new_window() -> Result<WebviewWindow, String> {
     match builder.build() {
         Ok(window) => {
             logging_error!(Type::Window, window.set_background_color(Some(background_color)));
-            logging_error!(Type::Window, window.eval(INITIAL_LOADING_OVERLAY));
             Ok(window)
         }
         Err(e) => Err(e.to_string()),

src-tauri/src/utils/resolve/window_script.rs

@@ -91,11 +91,3 @@ pub const WINDOW_INITIAL_SCRIPT: &str = r##"
 
     console.log('[Tauri] 窗口初始化脚本执行完成');
 "##;
-
-pub const INITIAL_LOADING_OVERLAY: &str = r"
-    const overlay = document.getElementById('initial-loading-overlay');
-    if (overlay) {
-        overlay.style.opacity = '0';
-        setTimeout(() => overlay.remove(), 300);
-    }
-";