styles: box-sizing reset + body overflow:hidden — kill the 2px-border overflow bug

#app has width:100vw and a 1px border with default content-box box-sizing,
so its rendered size is 100vw+2px which overflowed body → unwanted scrollbars.

src/styles.css

@@ -22,12 +22,15 @@
   color: var(--fg);
 }
 
+*, *::before, *::after { box-sizing: border-box; }
+
 html, body {
   margin: 0;
   padding: 0;
   width: 100%;
   height: 100%;
   background: transparent; /* honor Tauri transparent:true */
+  overflow: hidden;        /* viewport never scrolls; sections handle their own */
   user-select: none;
   -webkit-user-select: none;
 }