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_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);
-    }
-";