Use full available width for reader component (#618)

Due to horizontally centering the flex-box items via "alignItems", the items did not stretch anymore, which is the default value for "alignItems" and thus, did not take up the available width.

Regression introduced with ab0ecf4da0

src/screens/Reader.tsx

@@ -423,6 +423,7 @@ export function Reader() {
             }}
         >
             <PageNumber settings={settings} curPage={curPage} pageCount={chapter.pageCount} />
+            <Box sx={{ alignSelf: 'stretch' }}>
                 <ReaderComponent
                     key={chapter.id}
                     pages={pages}
@@ -437,5 +438,6 @@ export function Reader() {
                     prevChapter={loadPrevChapter}
                 />
             </Box>
+        </Box>
     );
 }