- 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.)
