Remove double scrollbar in Chrome PWA

html/body no longer scroll — overflow: hidden on both.
.main-content gets height: calc(100% - nav-h) and overflow-y: auto
so it's the single scroll container. The reader overlay is
position: fixed so it's unaffected.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/css/app.css

@@ -8,6 +8,11 @@
   padding: 0;
 }
 
+html, body {
+  height: 100%;
+  overflow: hidden;
+}
+
 :root {
   --bg: #000;
   --bg-card: transparent;
@@ -139,6 +144,8 @@ a:hover {
   max-width: 1100px;
   margin: 0 auto;
   padding: 1rem 1.5rem calc(var(--bar-h) + 2rem);
+  height: calc(100% - var(--nav-h));
+  overflow-y: auto;
 }
 
 /* =========================================================