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/globals.css

@@ -174,6 +174,7 @@
 html,
 body {
   height: 100%;
+  overscroll-behavior: none;
 }
 
 body {