Fix layout double-offset from persistent drawer

The sidebar Drawer reserved width in the flex container AND the main
content had margin-left for the same width, pushing content 440px right.
Removed the redundant margin-left and added a width transition to the
Drawer for smooth toggle animation.

web/src/components/Sidebar.tsx

@@ -64,6 +64,7 @@ export default function Sidebar() {
       sx={{
         width: open ? DRAWER_WIDTH : 0,
         flexShrink: 0,
+        transition: 'width 250ms ease',
         '& .MuiDrawer-paper': {
           width: DRAWER_WIDTH,
           boxSizing: 'border-box',