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/index.css

@@ -558,7 +558,11 @@ body {
   .reveal:focus-visible {
     opacity: 1;
   }
-  .group:hover .reveal-idle {
+  /* Fade the idle badge whenever its companion control is revealed — on hover AND on keyboard
+     focus-within (the `.reveal` trigger set above lists both), else the two overlap at the same
+     corner for a keyboard user. */
+  .group:hover .reveal-idle,
+  .group:focus-within .reveal-idle {
     opacity: 0;
   }
 }