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],
   );