hotfix: offline banner as bottom pill instead of full-width top bar

The top bar still blocked the trip planner's top nav on mobile even
after #808's padding trick — nav layouts that position their own
sticky headers were ignoring the --offline-banner-h offset, and the
bar looked alarming for what is usually a 2s blip.

Redesign as a small floating pill anchored bottom-center, hovering
above the mobile bottom nav (calc(var(--bottom-nav-h) + 16px)). No
layout shift anywhere, nothing ever covers the nav, and the pill
looks like a passing status chip rather than an error banner.

Reverts the body padding-top / navbar top offset introduced in #808
since they're no longer needed with the pill positioning.

client/src/components/Layout/Navbar.tsx

@@ -103,7 +103,6 @@ export default function Navbar({ tripTitle, tripId, onBack, showBack, onShare }:
       touchAction: 'manipulation',
       paddingTop: 'env(safe-area-inset-top, 0px)',
       height: 'var(--nav-h)',
-      top: 'var(--offline-banner-h, 0px)',
       transition: 'background 240ms cubic-bezier(0.23,1,0.32,1), backdrop-filter 240ms cubic-bezier(0.23,1,0.32,1), box-shadow 240ms cubic-bezier(0.23,1,0.32,1)',
     }} className="hidden md:flex items-center px-4 gap-4 fixed top-0 left-0 right-0 z-[200]">
       {/* Left side */}