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/Header.tsx

@@ -76,7 +76,9 @@ export default function Header({
             ? t("header.usageStats")
             : page === "playlists"
               ? t("header.account.playlists")
-              : t("header.channelManager")}
+              : page === "settings"
+                ? t("settings.title")
+                : t("header.channelManager")}
         </div>
       )}