Info page: show the app version, two-column layout on wide screens

An About panel shows the installed version (from Tauri at runtime, since
release builds stamp it at build time; dev builds are labelled as such).
On wide screens the page splits into panels in two columns — card
database, library stats and libraries on the left; sync and about on the
right — while narrow screens keep the single-column order. The sync panel
is now headed "Sync" and the Info page's summary "Library stats".

Closes #22

src/SyncPanel.tsx

@@ -208,7 +208,7 @@ export function SyncPanel({
 
   return (
     <>
-      <h2 style={{ margin: 0 }}>Your library</h2>
+      <h2 style={{ margin: 0 }}>Sync</h2>
 
       {!sync.signed_in ? (
         <>