feat(nav): restore scroll position per history entry, open forward views at the top

The app scrolls one persistent <main> (PageScroller); its saved offsets were keyed by a
content `scrollKey` that is the same for a Plex page's Discover and grid views, so opening
a Browse-by-genre tile inherited the scrolled-down Discover offset (landing mid-page) and
Back couldn't tell the two views apart.

Tie scroll memory to the history entry instead: every pushState that opens a main-content
view stamps a fresh `sfScroll` id into history.state (setPage, Plex sub-view open); the
initial/legacy entries get one lazily via replaceState. PageScroller now saves/restores per
`${entryId}:${scrollKey}` — the entry id makes Back restore exactly where you left an entry
and a forward navigation (new id) open at the top, while the scrollKey still resets a new
feed filter to the top within the same entry. A small nav bus lets a Plex sub-view open
(which doesn't re-render the scroller) trigger the restore; `history.scrollRestoration` is
set to manual since the browser can't see our inner scroller.

Verified live: genre tile opens the grid at the top and Back returns to the Discover scroll;
feed→plex→Back restores the feed scroll; a feed filter change still resets to the top.

Gate: tsc/lint/knip/prettier clean, 5 new nav-id unit tests, full vitest green.

frontend/src/components/NavigationProvider.tsx

@@ -14,6 +14,7 @@ import { pageTitleKey } from "../lib/pageMeta";
 import { setAccountRaw } from "../lib/storage";
 import { useAccountRehydrate } from "../lib/useAccountRehydrate";
 import { setNavigator } from "../lib/nav";
+import { nextNavId, notifyNav } from "../lib/history";
 import { channelViewFrom, loadInitialPage, PAGE_KEY, type ChannelView } from "./navigationInit";
 
 // Owns the app's navigation + browser-history state, lifted out of the App god component so pages
@@ -128,10 +129,11 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
       // (chan/v/pl) — a page switch resets to that module's root. Feed filters (owned by
       // FeedFiltersProvider) are left intact.
       window.history.pushState(
-        { sfPage: next },
+        { sfPage: next, sfScroll: nextNavId() },
         "",
         urlWithParams({ page: next === "feed" ? null : next, chan: null, v: null, pl: null }),
       );
+      notifyNav();
     },
     [markDirty],
   );

frontend/src/components/PageScroller.tsx

@@ -1,9 +1,12 @@
 import { useCallback, useLayoutEffect, useRef, type ReactNode } from "react";
 import { useScrollFade } from "../lib/useScrollFade";
+import { ensureNavId, peekNavId, subscribeNav } from "../lib/history";
 
-// Per-view saved scroll offsets. Module-level so a view's position survives its scroller
-// unmounting — opening a channel page swaps in a different shell/scroller — and is restored when
-// the same view returns. Keyed by the shell's `scrollKey`.
+// Saved scroll offsets, keyed by `${historyEntryId}:${scrollKey}`. The entry id ties an offset to the
+// exact history entry, so Back restores where you left it and a forward navigation (a new entry) opens
+// at the top; the scrollKey ties it to the logical content, so a new feed filter still starts at the
+// top WITHIN the same entry. Module-level so a view's position survives its scroller unmounting
+// (opening a channel page swaps in a different shell/scroller) and is restored when it returns.
 const savedScroll = new Map<string, number>();
 
 // Put the scroller back to `top`. The feed is virtualized, so on a remount the list needs a frame
@@ -85,23 +88,50 @@ export default function PageScroller({
     [fadeRef],
   );
 
+  // Live scrollKey for the event/nav-bus callbacks (which are bound once, not per scrollKey).
+  const scrollKeyRef = useRef(scrollKey);
+  scrollKeyRef.current = scrollKey;
+  const lastComposite = useRef<string | null>(null);
+  const cancelRef = useRef<() => void>(() => {});
+
+  // Restore (or reset to top) whenever the (history entry + content) identity changes — a page/sub-view
+  // navigation, a Back/Forward, or a feed filter change. Idempotent: a no-op when the composite is
+  // unchanged, so it's safe to call from several triggers.
+  const resolve = useCallback(() => {
+    const el = elRef.current;
+    if (!el || scrollKeyRef.current == null) return;
+    const key = `${ensureNavId()}:${scrollKeyRef.current}`;
+    if (key === lastComposite.current) return;
+    lastComposite.current = key;
+    cancelRef.current();
+    cancelRef.current = restoreScroll(el, savedScroll.get(key) ?? 0);
+  }, []);
+
+  // Trigger 1: mount + a content-key change (page switch, feed filter) — both re-render this component.
+  useLayoutEffect(() => {
+    resolve();
+  }, [scrollKey, resolve]);
+
+  // Trigger 2: Back/Forward (popstate) and a Plex sub-view open (nav bus) — neither necessarily
+  // re-renders this component. Also records the position continuously (not at unmount): the scroller's
+  // scrollTop is unreliable by the time an unmount cleanup runs, so a save-on-scroll survives teardown.
   useLayoutEffect(() => {
     const el = elRef.current;
-    if (!el || scrollKey == null) return;
-    const key = scrollKey;
-    const want = savedScroll.get(key) ?? 0;
-    const cancelRestore = restoreScroll(el, want);
-    // Record position continuously (not at unmount): the scroller's scrollTop is unreliable by the
-    // time an unmount cleanup runs, and a save-on-scroll survives whatever tears down last.
+    if (!el) return;
     const onScroll = () => {
-      savedScroll.set(key, el.scrollTop);
+      if (scrollKeyRef.current == null) return;
+      savedScroll.set(`${peekNavId()}:${scrollKeyRef.current}`, el.scrollTop);
     };
     el.addEventListener("scroll", onScroll, { passive: true });
+    window.addEventListener("popstate", resolve);
+    const unsubNav = subscribeNav(resolve);
     return () => {
-      cancelRestore();
+      cancelRef.current();
       el.removeEventListener("scroll", onScroll);
+      window.removeEventListener("popstate", resolve);
+      unsubNav();
     };
-  }, [scrollKey]);
+  }, [resolve]);
 
   return (
     <main