feat: global navigation progress bar

Most data routes run their load with ssr = false, so SvelteKit holds the
previous page on screen while the next one's data resolves — with no
feedback. NavProgress, mounted once in the root layout and driven by the
$navigating store, shows a thin top bar that trickles while a navigation is
pending and fades out on completion. Degrades to a static visible bar under
prefers-reduced-motion. New --z-nav-progress token sits above the fixed
header but below modals/toasts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -9,6 +9,7 @@
     import { theme } from '$lib/theme.svelte';
     import AppBar from '$lib/components/AppBar.svelte';
     import BottomNav, { type BottomNavTab } from '$lib/components/BottomNav.svelte';
+    import NavProgress from '$lib/components/NavProgress.svelte';
     import IconButton from '$lib/components/IconButton.svelte';
     import Upload from '@lucide/svelte/icons/upload';
     import UserCircle from '@lucide/svelte/icons/user-circle';
@@ -198,6 +199,8 @@
     <title>{layoutTitle}</title>
 </svelte:head>
 
+<NavProgress />
+
 {#if showMobileChrome}
     <div class="mobile-app-bar-wrap" bind:this={appBarEl}>
         <AppBar title="Mangalord" testid="mobile-app-bar" />