feat(nav): Settings as a page module (Design B)

Move Settings out of the right-side overlay into a left-nav page (page='settings'),
so it opens where you're already looking — no cross-screen mouse travel. The Settings
rail item now sets the page (with active highlight) instead of opening a dialog; the
panel is refactored to an in-flow .glass card (keeps the frosted look over the ambient
backdrop), with the page title shown in the header. Removed the overlay + Esc/backdrop
close path.

frontend/src/components/NavSidebar.tsx

@@ -23,13 +23,11 @@ export default function NavSidebar({
   me,
   page,
   setPage,
-  onOpenSettings,
   onOpenAbout,
 }: {
   me: Me;
   page: Page;
   setPage: (p: Page) => void;
-  onOpenSettings: () => void;
   onOpenAbout: () => void;
 }) {
   const { t } = useTranslation();
@@ -115,9 +113,14 @@ export default function NavSidebar({
 
       <div className="mt-2 pt-2 border-t border-border flex flex-col gap-1">
         <button
-          onClick={onOpenSettings}
+          onClick={() => setPage("settings")}
           title={collapsed ? t("header.account.settings") : undefined}
-          className={`${rowBase} ${collapsed ? "justify-center px-0" : ""} text-muted hover:text-fg hover:bg-card`}
+          aria-current={page === "settings" ? "page" : undefined}
+          className={`${rowBase} ${collapsed ? "justify-center px-0" : ""} ${
+            page === "settings"
+              ? "bg-accent text-accent-fg"
+              : "text-muted hover:text-fg hover:bg-card"
+          }`}
         >
           <Settings className="w-[18px] h-[18px] shrink-0" />
           {!collapsed && <span className="truncate">{t("header.account.settings")}</span>}