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/lib/stores/auth.ts

@@ -569,3 +569,5 @@ export const securityWarning = derived(auth, ($auth) => $auth.securityWarning);
 export const authError = derived(auth, ($auth) => $auth.error);
 export const isVerifying = derived(auth, ($auth) => $auth.isVerifying);
 export const sessionVerified = derived(auth, ($auth) => $auth.sessionVerified);
+export const serverName = derived(auth, ($auth) => $auth.serverName);
+export const serverUrl = derived(auth, ($auth) => $auth.serverUrl);