Frontend V2 Ui Tweaks (#4590)

* Top Controls only show when files > 0
* Moved content down so top controls don't obscure
* Viewer background set to match workbench and shadow around pages added
so that page boundaries are visible
* unsaved-changes modal rework

---------

Co-authored-by: Connor Yoh <[email protected]>

frontend/src/components/viewer/LocalEmbedPDF.tsx

@@ -274,7 +274,7 @@ export function LocalEmbedPDF({ file, url, enableAnnotations = false, onSignatur
         <GlobalPointerProvider>
           <Viewport
             style={{
-              backgroundColor: 'var(--bg-surface)',
+              backgroundColor: 'var(--bg-background)',
               height: '100%',
               width: '100%',
               maxHeight: '100%',
@@ -299,7 +299,8 @@ export function LocalEmbedPDF({ file, url, enableAnnotations = false, onSignatur
                       userSelect: 'none',
                       WebkitUserSelect: 'none',
                       MozUserSelect: 'none',
-                      msUserSelect: 'none'
+                      msUserSelect: 'none',
+                      boxShadow: '0 2px 8px rgba(0, 0, 0, 0.15)'
                     }}
                     draggable={false}
                     onDragStart={(e) => e.preventDefault()}