fix(header): restore inner scroll broken by the floating-header wrapper

The wrapper that pads content below the fixed header lacked min-h-0, so as a
flex child with min-height:auto it grew to its content's height instead of
staying bounded to the viewport. That let the whole document scroll (the fixed
header/nav stayed put while content ran off-screen). Add min-h-0 so the wrapper
stays viewport-height and the inner <main> keeps its own overflow scroll.

frontend/src/App.tsx

@@ -819,7 +819,7 @@ export default function App() {
             wrapper clears it with a top pad — EXCEPT on Playlists, whose own left rail must reach
             the very top (it pads only its content pane internally instead). */}
         <div
-          className={`flex-1 min-w-0 flex flex-col ${page === "playlists" ? "" : "pt-[var(--hdr-h)]"}`}
+          className={`flex-1 min-w-0 min-h-0 flex flex-col ${page === "playlists" ? "" : "pt-[var(--hdr-h)]"}`}
         >
         {meQuery.data!.is_demo && <DemoBanner />}
         <VersionBanner onOpen={() => openReleaseNotes(CURRENT_VERSION)} />