fix(window): keep Linux window opaque so it can't turn invisible (#3682) (#4904)

On Linux the window was created fully transparent to draw rounded corners
(#1982), but on WebKitGTK a transparent window composites as transparent
whenever its web process is too busy to repaint damaged regions (for example
during a library backup). Interacting with the app then makes it appear to
turn invisible, showing the desktop through the window.

Make the window opaque everywhere: the main window in lib.rs and the
reader/extra windows in nav.ts. Drop the rounded-window treatment
(hasRoundedWindow=false) so no rounded 1px border floats on the now square
opaque window, and give the Linux loading placeholders a solid background.
An opaque window retains its last painted frame instead of going invisible;
the tradeoff is square corners on Linux.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/readest-app/src/app/reader/components/Reader.tsx

@@ -165,7 +165,7 @@ const Reader: React.FC<{ ids?: string }> = ({ ids }) => {
       </Suspense>
     </div>
   ) : (
-    <div className={clsx('full-height', !appService?.isLinuxApp && 'bg-base-100')}></div>
+    <div className='full-height bg-base-100'></div>
   );
 };