refactor(frontend): satisfy noUncheckedIndexedAccess (S3b)

Enable noUncheckedIndexedAccess and fix all 98 call sites it surfaced across 18
files. Every fix is behaviour-preserving — the flag flagged reads TypeScript
couldn't prove in-bounds, all of which were already guarded by a length/index check,
a findIndex result, or a non-empty invariant:

- read-once so a ternary's narrowing sticks (Feed overrides, linkify mention);
- non-null assertion AFTER an existing guard (PlayerModal/VideoEditor queue+cut-list
  indexing, modules step, useUndoable stacks) with a comment stating the invariant;
- nullish fallback where undefined is a real possibility (GlassTuner slider ?? def,
  ConfigPanel active group ?? [], descriptionLinks id ?? null);
- optional chaining where the entry genuinely can be absent (PlexBrowse IO entry);
- a non-empty tuple type for the RELATIVE_UNITS constant (encodes "always has [0]").

The single highest-leverage fix: PlayerModal's `active` became `queue?.[index] ??
video` (always Video), clearing 34 of the 45 errors in that file at once. Verified:
tsc/eslint/prettier clean, 56 vitest + 34 pytest + 17/17 e2e green (the e2e suite
exercises the feed/player/channel components touched here).

frontend/src/components/PlexPlayer.tsx

@@ -54,7 +54,7 @@ function audioOrdForLang(streams: AudioStream[], lang: string): number | null {
   // absent (-1) or already the FIRST track (0, which the backend plays by default) → no override.
   // (Match by index, NOT the `default` flag — some files mark a non-first track default, which made
   // the restored language silently fall back to track 0.)
-  return i <= 0 ? null : streams[i].ord;
+  return i <= 0 ? null : streams[i]!.ord; // i > 0 here, so streams[i] exists
 }
 function subOrdForLang(subs: SubStream[], lang: string): number | null {
   if (!lang) return null; // "" = subtitles off
@@ -666,14 +666,14 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
     const streams = detail?.audio_streams ?? [];
     if (streams.length < 2) return;
     const idx = streams.findIndex((s) => s.ord === (audioRef.current ?? 0));
-    const next = streams[(idx + 1) % streams.length];
+    const next = streams[(idx + 1) % streams.length]!; // length >= 2 (guarded), so in-bounds
     changeAudio(next.ord === 0 ? null : next.ord);
   }, [detail, changeAudio]);
   const cycleSubtitle = useCallback(() => {
     const subs = (detail?.subtitle_streams ?? []).filter((s) => s.text);
     const order: (number | null)[] = [null, ...subs.map((s) => s.ord)];
     const i = order.findIndex((o) => o === subOrd);
-    changeSubtitle(order[(i + 1) % order.length]);
+    changeSubtitle(order[(i + 1) % order.length]!); // order always has [null, ...] → non-empty
   }, [detail, subOrd, changeSubtitle]);
   // Audio A/V-sync offset: persist immediately (for the slider) but debounce the session restart so
   // dragging doesn't respawn ffmpeg on every step.