fix(e4): address deep-review findings — bookmark popstate, focus trap, touch a11y

Deep-review (6 dimensions, 3-verifier adversarial pass) surfaced 7 findings, all
verified; fixed here.

- NavigationProvider + history.ts (medium, correctness): the popstate handlers
  restored the open channel / Plex sub-view from history.state ONLY, but a fresh
  ?chan / ?v bookmark lives in the URL with no marker in state — so any overlay's
  programmatic history.back() (closing a modal/player) silently closed the
  bookmarked channel or collapsed the sub-view to the grid. Give onPop the same
  URL fallback the initializers already use (history.ts extracts a shared readView
  so seed and popstate can't diverge again).
- plexCards SeasonCard (medium, a11y): the "In progress" season badge was a
  reveal-idle swap-badge → display:none on touch, and a season has no progress bar,
  so an in-progress season was indistinguishable from unwatched on touch. Moved it
  to the top-left (the toggle owns top-right) and made it always visible.
- plexCards watched toggle (low, a11y / WCAG 1.4.1): watched state was color-only
  on touch (green vs white CheckCircle2). New WatchedIcon distinguishes by SHAPE —
  CheckCircle2 (watched) vs an empty Circle (not) — used by all three card toggles.
- Modal.tsx (low, a11y): the focus trap leaked on Shift+Tab when focus rested on
  the panel container itself (Node.contains(self) is true, so the pull-back branch
  never ran). Guard now also checks `active === panel`.
- index.css (low, a11y): .reveal-idle faded only on .group:hover though .reveal is
  revealed on hover AND focus-within → the badge overlapped the control on keyboard
  focus. Added the focus-within fade.
- PlexBrowse.tsx (low, correctness): the ?v=playlist decoder accepted a non-integer
  id (Number.isFinite); tightened to Number.isInteger(n) && n > 0, matching the ?pl
  guard.

Gate green (tsc, knip, lint 0-err, 247 vitest). Self-E2E (localdev): ?v=show and
?chan bookmarks survive a modal open+close (were collapsing/closing before); Modal
Shift+Tab from the panel stays inside; in-progress season badge visible on touch;
watched toggle shows an empty-circle-vs-check shape difference; 0 console errors.

frontend/src/lib/history.ts

@@ -32,8 +32,12 @@ export function useHistorySubview<T>(
   open: (next: T) => void;
   back: () => void;
 } {
-  const [view, setView] = useState<T>(() => {
-    // Reload of an in-app entry: history.state._sub. Fresh bookmark: decode the ?<key> param.
+  // Resolve the current entry's sub-view: history.state._sub for an in-app entry, else decode the
+  // ?<key> URL param for a fresh bookmark (whose entry carries the param but no _sub), else root.
+  // ⚠️ The seed AND the popstate handler must use THIS same fallback: a popstate can land on a
+  // bookmark entry (URL-only, no _sub) — e.g. after an overlay's programmatic history.back() — and
+  // reading history.state alone would wrongly collapse the bookmarked sub-view to root.
+  const readView = (): T => {
     const fromState = window.history.state?._sub as T | undefined;
     if (fromState != null) return fromState;
     if (url) {
@@ -42,13 +46,15 @@ export function useHistorySubview<T>(
       if (decoded != null) return decoded;
     }
     return root;
-  });
+  };
+  const readViewRef = useRef(readView);
+  readViewRef.current = readView;
+  const [view, setView] = useState<T>(readView);
   useEffect(() => {
-    // Back/Forward restores the sub-view from history.state (the browser restores each entry's URL too).
-    const onPop = () => setView((window.history.state?._sub as T) ?? root);
+    // Back/Forward restores the sub-view from the landed entry (state OR its restored ?<key> URL).
+    const onPop = () => setView(readViewRef.current());
     window.addEventListener("popstate", onPop);
     return () => window.removeEventListener("popstate", onPop);
-    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, []);
   return {
     view,