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/components/plexCards.tsx

@@ -1,5 +1,5 @@
 import { useTranslation } from "react-i18next";
-import { CheckCircle2, Film, Info, ListPlus, Play, Tv2 } from "lucide-react";
+import { CheckCircle2, Circle, Film, Info, ListPlus, Play, Tv2 } from "lucide-react";
 import { type CSSProperties } from "react";
 import { formatRuntime } from "../lib/format";
 import type { PlexCard, PlexCastMember, PlexFilters, PlexSeasonDetail } from "../lib/api";
@@ -24,6 +24,18 @@ const PLAYABLE_TINT: Record<string, string> = {
 const swapBadgeReveal = (hasControl: boolean): string =>
   hasControl ? "reveal-idle" : "group-hover:opacity-0";
 
+// Watched-toggle icon. Distinguishes state by SHAPE (a filled check vs an empty circle), not color
+// alone — on touch the toggle is the only surviving watched indicator (the text pill is hidden), so a
+// green-vs-white CheckCircle2 would be a use-of-color (WCAG 1.4.1) gap. Watched = CheckCircle2 (green),
+// not-watched = Circle (empty, white).
+function WatchedIcon({ watched }: { watched: boolean }) {
+  return watched ? (
+    <CheckCircle2 className="w-4 h-4 text-success" />
+  ) : (
+    <Circle className="w-4 h-4 text-white" />
+  );
+}
+
 export function PlexPosterCard({
   card,
   onOpen,
@@ -90,9 +102,7 @@ export function PlexPosterCard({
             title={card.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
             className="absolute top-1.5 right-1.5 p-1 rounded-full bg-black/60 reveal hover:bg-black/80 transition"
           >
-            <CheckCircle2
-              className={`w-4 h-4 ${card.status === "watched" ? "text-success" : "text-white"}`}
-            />
+            <WatchedIcon watched={card.status === "watched"} />
           </button>
         )}
         {/* Media info page (movies/episodes). */}
@@ -219,7 +229,7 @@ export function SeasonCard({
             }
             className="absolute top-1.5 right-1.5 p-1 rounded-full bg-black/60 reveal hover:bg-black/80 transition"
           >
-            <CheckCircle2 className={`w-4 h-4 ${watched ? "text-success" : "text-white"}`} />
+            <WatchedIcon watched={watched} />
           </button>
         )}
         {/* Add whole season to a playlist, on hover. */}
@@ -244,12 +254,12 @@ export function SeasonCard({
             {t("plex.watched")}
           </span>
         )}
+        {/* In-progress lives at the top-LEFT (the watched-toggle owns the top-right) and stays visible
+            on touch: unlike movie/episode cards, a season has no progress bar, and the always-shown
+            toggle reads as unwatched for a partial season — so this badge is the sole in-progress cue
+            and must not be a reveal-idle swap-badge (which display:none-s on touch). */}
         {se.status === "in_progress" && (
-          <span
-            className={`pointer-events-none absolute top-1.5 right-1.5 text-3xs bg-black/75 text-accent px-1.5 py-0.5 rounded-sm ${swapBadgeReveal(
-              !!onToggleWatched,
-            )}`}
-          >
+          <span className="pointer-events-none absolute top-1.5 left-1.5 text-3xs bg-black/75 text-accent px-1.5 py-0.5 rounded-sm">
             {t("plex.inProgress")}
           </span>
         )}
@@ -430,9 +440,7 @@ export function EpisodeCard({
             title={ep.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
             className="absolute top-1.5 right-1.5 p-1 rounded-full bg-black/60 reveal hover:bg-black/80 transition"
           >
-            <CheckCircle2
-              className={`w-4 h-4 ${ep.status === "watched" ? "text-success" : "text-white"}`}
-            />
+            <WatchedIcon watched={ep.status === "watched"} />
           </button>
         )}
         {ep.status === "watched" && (