Desktop library pass: alignment, card weight, sidebar and theme

Layout (desktop / lg+ only):
- 40px top padding above the page title.
- Deck grid uses minmax(320px, 1fr), so two cards fill the content width
  and share a right edge with the header (same max-width container).
- Group headings only render once real folders exist; with none, a single
  quiet "All decks" label replaces "Unorganized".
- Import JSON becomes a plain text button (mobile keeps the pill).

Deck cards (desktop):
- The whole card is a link; Study is a compact secondary button instead
  of a full-width solid pill.
- The ... control is a 32px ghost button with a real ellipsis icon,
  aligned to the title's centre.
- Meta line always shows the same fields (activities · runs / Not played
  yet); the score is a separate quiet "Best 53%" stat.
- 12px rhythm between title/meta/tags/summary, 16px before the actions.
- Hover: lighter surface, stronger border, soft shadow and a 2px lift in
  light mode (pointer devices only, so touch has no sticky hover).

Sidebar (desktop rail):
- All decks + Unorganized are views above the FOLDERS label; only
  user-created folders sit under it.
- Count only on All decks. The mobile drawer is unchanged.

Theme:
- Dark: sidebar is a step darker (#181818 to #141414 vs #212121 page).
- Light: cards get a slightly stronger border (#e2e2e2) and a soft
  shadow; the sidebar divider and edge shadow are stronger too.
- Medium difficulty darkened to #7a5300 (6.2:1 on the light chip).

Mobile is untouched: layout changes are behind lg: (or the desktop prop);
theme tweaks are shared tokens.

Verified: key-gate 49/49, install sheet 64/64, retry 14/14, zoom 6/6,
deck chrome 15/15, deck fixes 15/15, final ux 12/12, contrast 20/20,
security 8/8, pwa 16/16.

app/page.tsx

@@ -13,7 +13,7 @@ export default function HomePage() {
         <MobileSidebar />
         <main className="flex min-h-dvh min-w-0 flex-1 flex-col">
           <AppHeader />
-          <div className="mx-auto w-full max-w-6xl flex-1 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 lg:pt-10">
             <LibraryScreen />
           </div>
           <TabBar />