fix(nav): Back steps through sub-views and closes modals before leaving a page

In-app history only tracked the top-level page, so Back from a module sub-view
(e.g. a Messages thread) or with a modal open jumped straight to the previous
module. Add two history primitives: useHistorySubview (a module's sub-view rides
in history.state, so Back returns to its root first) and useBackToClose (a
mounted overlay occupies one history entry; Back closes the topmost, nesting-safe
so a button-close doesn't trip the modals underneath). Apply to the Messages page
views and to PlayerModal + the shared Modal. setPage now pushes a clean entry so
each page starts at its root.

frontend/src/App.tsx

@@ -186,7 +186,8 @@ export default function App() {
       // Push an in-app history entry so the browser/mouse Back button steps through pages
       // instead of leaving the app (e.g. back to the OAuth redirect). The URL stays clean —
       // the page rides in history.state, not the query string (filters never go in the URL).
-      window.history.pushState({ ...window.history.state, sfPage: next }, "");
+      // A fresh { sfPage } (no carried-over _sub/_ov markers) so each page starts at its root.
+      window.history.pushState({ sfPage: next }, "");
     };
     // Guard leaving the Settings page with unsaved preference changes.
     if (page === "settings" && prefsDirtyRef.current) {