feat(chrome): shared account menu and global app header

Move account actions (Settings, Downloads, Display preferences, Sign
out) out of the library-only header into a shared AccountMenu anchored in
a global AppHeader, available on every authenticated non-immersive
screen. Add a layoutShell helper deciding where chrome shows, expose
serverName/serverUrl auth stores, and a display view-mode preference. The
settings page also gains the UR-053 WiFi-only toggle.

TRACES: UR-054 | DR-075, DR-076, DR-077

src/routes/+layout.svelte

@@ -16,16 +16,22 @@
   import Toast from "$lib/components/Toast.svelte";
   import SleepTimerModal from "$lib/components/player/SleepTimerModal.svelte";
   import BottomUi from "$lib/components/BottomUi.svelte";
+  import AppHeader from "$lib/components/AppHeader.svelte";
   import { isInitialized, pendingSyncCount, isAndroid, showSleepTimerModal } from "$lib/stores/appState";
   import {
     showBottomNav as computeShowBottomNav,
     showGlobalMiniPlayer as computeShowGlobalMiniPlayer,
+    showGlobalHeader as computeShowGlobalHeader,
     routeOwnsLayout as computeRouteOwnsLayout,
   } from "$lib/utils/layoutShell";
   import { registerNavigationTracking } from "$lib/utils/navigation";
+  import { startNetworkReporting } from "$lib/services/networkType";
 
   let { children } = $props();
 
+  /** Teardown for the network-transport reporter (WiFi-only gate). */
+  let stopNetworkReporting: (() => void) | null = null;
+
   // Track in-app navigation depth so the header "back" affordance knows when a
   // real in-app Back exists (vs. a stale WebView stack after a background /
   // restore). Must run during component init — afterNavigate needs a component
@@ -47,6 +53,13 @@
   );
   const showGlobalMiniPlayer = $derived(computeShowGlobalMiniPlayer({ pathname }));
 
+  // The shared header (with the account menu) is rendered by the root on every
+  // authenticated non-immersive route that doesn't own its own layout. Library
+  // renders its own AppHeader; settings/player/login get none. (UR-054)
+  const showGlobalHeader = $derived(
+    computeShowGlobalHeader({ pathname, isAuthenticated: $isAuthenticated })
+  );
+
   // Library/settings/player/login own their own full-height flex column
   // (header + scroller + their own in-flow BottomUi), so the root just clips
   // and lets them manage layout. Every other route renders into the root's
@@ -76,6 +89,11 @@
     // Initialize download event listener
     await initDownloadEvents();
 
+    // Report the network transport to the backend and keep it current, so the
+    // WiFi-only download gate has real data to act on (UR-053). No-op on
+    // desktop, where the backend defaults to unmetered.
+    stopNetworkReporting = startNetworkReporting();
+
     // Prime the downloads store from the DB. The store starts empty each launch,
     // and download badges (e.g. AlbumDownloadButton) derive purely from it, so
     // without an initial refresh a previously-downloaded album shows as
@@ -102,6 +120,7 @@
   });
 
   onDestroy(() => {
+    stopNetworkReporting?.();
     cleanupPlayerEvents();
     cleanupDownloadEvents();
     connectivity.stopMonitoring();
@@ -189,6 +208,11 @@
         {@render children()}
       </div>
     {:else}
+      <!-- Shared header (account menu, desktop nav) as a flex-shrink-0 sibling
+           above the scroller, so it never eats into the scroller's bounds. -->
+      {#if showGlobalHeader}
+        <AppHeader />
+      {/if}
       <!-- Scroller is flex-1/min-h-0; the in-flow BottomUi below is a flex
            sibling, so the list is physically bounded above it and can never
            render behind it. No measurement, no reserved padding. -->