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/PlayerModal.tsx

@@ -124,7 +124,7 @@ export default function PlayerModal({
   // where a just-watched item could drop out and snap playback to queue[0]).
   let index = queue ? queue.findIndex((v) => v.id === playingId) : 0;
   if (index < 0) index = 0;
-  const active = queue && queue[index] ? queue[index] : video;
+  const active = queue?.[index] ?? video; // the queue item at index, else the opened video (never undefined)
   const hasQueue = !!queue && queue.length > 1;
   // startAt only applies to the item we opened with; advanced items resume their own pos.
   const resumeAt =
@@ -242,11 +242,11 @@ export default function PlayerModal({
   navRef.current = { queue, index };
   const goPrev = () => {
     const { queue: q, index: i } = navRef.current;
-    if (q && i > 0) setPlayingId(q[i - 1].id);
+    if (q && i > 0) setPlayingId(q[i - 1]!.id);
   };
   const goNext = () => {
     const { queue: q, index: i } = navRef.current;
-    if (q && i < q.length - 1) setPlayingId(q[i + 1].id);
+    if (q && i < q.length - 1) setPlayingId(q[i + 1]!.id);
   };
   // Relative seek within the current video (plain Arrow keys), matching YouTube's ±5s.
   const nudgeSeek = (delta: number) => {
@@ -277,12 +277,12 @@ export default function PlayerModal({
     );
   };
   const cycleAuto = () => {
-    const next = AUTO_MODES[(AUTO_MODES.indexOf(autoMode) + 1) % AUTO_MODES.length];
+    const next = AUTO_MODES[(AUTO_MODES.indexOf(autoMode) + 1) % AUTO_MODES.length]!;
     setAutoMode(next);
     persistPref({ playerAutoAdvance: next });
   };
   const cycleLoop = () => {
-    const next = LOOP_MODES[(LOOP_MODES.indexOf(loopMode) + 1) % LOOP_MODES.length];
+    const next = LOOP_MODES[(LOOP_MODES.indexOf(loopMode) + 1) % LOOP_MODES.length]!;
     setLoopMode(next);
     persistPref({ playerLoop: next });
   };
@@ -303,21 +303,21 @@ export default function PlayerModal({
     if (!q || q.length === 0) return;
     const wrap = l === "all";
     if (a === "next") {
-      if (i < q.length - 1) setPlayingId(q[i + 1].id);
+      if (i < q.length - 1) setPlayingId(q[i + 1]!.id);
       else if (wrap) {
         if (q.length === 1) replayCurrent();
-        else setPlayingId(q[0].id);
+        else setPlayingId(q[0]!.id);
       }
     } else if (a === "prev") {
-      if (i > 0) setPlayingId(q[i - 1].id);
+      if (i > 0) setPlayingId(q[i - 1]!.id);
       else if (wrap) {
         if (q.length === 1) replayCurrent();
-        else setPlayingId(q[q.length - 1].id);
+        else setPlayingId(q[q.length - 1]!.id);
       }
     } else if (a === "random" && q.length > 1) {
       let r = i;
       while (r === i) r = Math.floor(Math.random() * q.length);
-      setPlayingId(q[r].id);
+      setPlayingId(q[r]!.id);
     } else if (a === "random" && wrap) {
       replayCurrent();
     }