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) {

frontend/src/components/Messages.tsx

@@ -6,6 +6,7 @@ import { api, type Conversation, type MessageUser } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { relativeTime } from "../lib/format";
 import { partnerPub, SYSTEM_ID, useKeyState } from "../lib/messaging";
+import { useHistorySubview } from "../lib/history";
 import { openChat } from "../lib/chatDock";
 import * as e2ee from "../lib/e2ee";
 import Avatar from "./Avatar";
@@ -17,7 +18,9 @@ const POLL_MS = 20000;
 type View = "list" | "new" | { partnerId: number; name?: string; avatar?: string | null; system?: boolean };
 
 export default function Messages({ meId }: { meId: number }) {
-  const [view, setView] = useState<View>("list");
+  // Sub-views live in browser history, so Back returns to the conversation list before leaving
+  // the module (open() pushes an entry; back() = history.back()).
+  const { view, open, back } = useHistorySubview<View>("list");
 
   if (typeof view === "object") {
     return (
@@ -27,18 +30,18 @@ export default function Messages({ meId }: { meId: number }) {
         isSystem={!!view.system}
         seedName={view.name}
         seedAvatar={view.avatar}
-        onBack={() => setView("list")}
+        onBack={back}
       />
     );
   }
   if (view === "new") {
-    return <Directory onPick={(u) => setView({ partnerId: u.id, name: u.name, avatar: u.avatar_url })} onBack={() => setView("list")} />;
+    return <Directory onPick={(u) => open({ partnerId: u.id, name: u.name, avatar: u.avatar_url })} onBack={back} />;
   }
   return (
     <ConversationList
       meId={meId}
-      onOpen={(c) => setView({ partnerId: c.partner.id, name: c.partner.name, avatar: c.partner.avatar_url, system: c.partner.id === SYSTEM_ID })}
-      onNew={() => setView("new")}
+      onOpen={(c) => open({ partnerId: c.partner.id, name: c.partner.name, avatar: c.partner.avatar_url, system: c.partner.id === SYSTEM_ID })}
+      onNew={() => open("new")}
     />
   );
 }

frontend/src/components/Modal.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useRef, type ReactNode } from "react";
 import { createPortal } from "react-dom";
 import { X } from "lucide-react";
+import { useBackToClose } from "../lib/history";
 
 // Stack of open modals so ESC only closes the topmost one — e.g. an error dialog over the
 // tag editor: pressing ESC dismisses just the error and returns to the editor underneath.
@@ -19,6 +20,9 @@ export default function Modal({
   children: ReactNode;
   maxWidth?: string;
 }) {
+  // Browser/mouse Back closes the topmost modal instead of navigating away.
+  useBackToClose(onClose);
+
   // Keep a stable handler across renders so the stack id is assigned once per mount.
   const onCloseRef = useRef(onClose);
   onCloseRef.current = onClose;

frontend/src/components/PlayerModal.tsx

@@ -8,6 +8,7 @@ import Avatar from "./Avatar";
 import AddToPlaylist from "./AddToPlaylist";
 import { api, type Video } from "../lib/api";
 import { channelYouTubeUrl, formatDuration, formatViews, relativeTime } from "../lib/format";
+import { useBackToClose } from "../lib/history";
 
 // Turn a description into clickable nodes:
 //  - bare timestamps (mm:ss / hh:mm:ss) → seek the inline player
@@ -203,6 +204,8 @@ export default function PlayerModal({
 }) {
   const { t, i18n } = useTranslation();
   const qc = useQueryClient();
+  // Browser/mouse Back closes the player instead of leaving the page.
+  useBackToClose(onClose);
   // Precise upload date shown inline after the relative time (e.g. "9 yr ago · 12 Jan 2017").
   const fullDate = (d: string | null | undefined) =>
     d

frontend/src/lib/history.ts

@@ -0,0 +1,76 @@
+// In-app browser-history integration so Back/Forward step through a module's sub-views and
+// overlays before leaving the page — not straight back to the previous module.
+//
+// Two primitives share the existing page-history (App stamps `sfPage` in history.state):
+//   - useHistorySubview: a module's active sub-view rides in history.state._sub, so Back inside
+//     a module (e.g. Messages thread → list) returns to the parent view first.
+//   - useBackToClose: while an overlay/modal is mounted it occupies one history entry; Back
+//     closes the topmost one. A module-level stack keeps nesting correct — closing one modal by
+//     its own button pops exactly its entry without tripping the modals underneath.
+//
+// setPage() pushes a clean { sfPage } entry (dropping _sub/_ov), so each page starts at its root.
+import { useEffect, useRef, useState } from "react";
+
+// --- Sub-views (value carried in history.state._sub) ---------------------------------------
+export function useHistorySubview<T>(root: T): {
+  view: T;
+  open: (next: T) => void;
+  back: () => void;
+} {
+  const [view, setView] = useState<T>(() => (window.history.state?._sub as T) ?? root);
+  useEffect(() => {
+    const onPop = () => setView((window.history.state?._sub as T) ?? root);
+    window.addEventListener("popstate", onPop);
+    return () => window.removeEventListener("popstate", onPop);
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, []);
+  return {
+    view,
+    open: (next: T) => {
+      setView(next);
+      window.history.pushState({ ...window.history.state, _sub: next }, "");
+    },
+    back: () => window.history.back(),
+  };
+}
+
+// --- Overlays / modals (one history entry each, nesting-safe) -------------------------------
+const overlayStack: number[] = [];
+let overlayCounter = 0;
+// Set while we pop our OWN entry programmatically (button/ESC close) so the other overlays'
+// popstate handlers don't mistake it for a user Back and close themselves too.
+let suppressPop = false;
+
+/** While the calling component is mounted, Back (or the browser/mouse back button) closes it via
+ * `onClose` instead of navigating away. Mount the component only while the overlay is open. */
+export function useBackToClose(onClose: () => void): void {
+  const cb = useRef(onClose);
+  cb.current = onClose;
+  useEffect(() => {
+    const token = ++overlayCounter;
+    overlayStack.push(token);
+    window.history.pushState({ ...window.history.state, _ov: token }, "");
+    const onPop = () => {
+      if (suppressPop) {
+        suppressPop = false;
+        return;
+      }
+      if (overlayStack[overlayStack.length - 1] === token) {
+        overlayStack.pop();
+        cb.current();
+      }
+    };
+    window.addEventListener("popstate", onPop);
+    return () => {
+      window.removeEventListener("popstate", onPop);
+      const idx = overlayStack.lastIndexOf(token);
+      if (idx !== -1) {
+        // Closed programmatically (not via Back): drop our own history entry, and tell the
+        // remaining overlays' handlers to ignore the resulting popstate.
+        overlayStack.splice(idx, 1);
+        suppressPop = true;
+        window.history.back();
+      }
+    };
+  }, []);
+}