Screens that already fit no longer scroll or rubber-band

- API-key gate is now a fixed, out-of-flow overlay (inset-0, overflow
  hidden, overscroll-none) with its own internal scroller, so the document
  has no scrollable height on phones. It can't be panned or bounced.
- Home/create mains are flex columns with min-h-dvh and a flex-1 content
  area, and the empty state's mobile padding was trimmed, so a short/empty
  library is exactly viewport height instead of overflowing by ~76px.
  Long pages (the create form, decks) still scroll normally.
- html/body get overscroll-behavior: none to stop pull-to-refresh and
  rubber-banding on fitting screens.

Verified with a scroll probe (gate and empty library: scrollHeight ==
viewport; create still scrolls) plus zoom 6/6, install sheet 64/64,
pwa 16/16, final-ux 12/12, retry 14/14, security 8/8, contrast 20/20.

app/page.tsx

@@ -11,9 +11,9 @@ export default function HomePage() {
         <Sidebar />
         <SidebarOffset />
         <MobileSidebar />
-        <main className="min-w-0 flex-1">
+        <main className="flex min-h-dvh min-w-0 flex-1 flex-col">
           <AppHeader />
-          <div className="mx-auto w-full max-w-6xl px-4 pb-tabbar lg:px-10 lg:pb-16">
+          <div className="mx-auto w-full max-w-6xl flex-1 px-4 pb-tabbar lg:px-10 lg:pb-16">
             <LibraryScreen />
           </div>
           <TabBar />