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/lib/useUndoable.ts

@@ -54,7 +54,7 @@ export function useUndoable<T>(
   const undo = useCallback(() => {
     const s = ref.current;
     if (!s.past.length) return;
-    const prev = s.past[s.past.length - 1];
+    const prev = s.past[s.past.length - 1]!; // past is non-empty (guarded above)
     ref.current = {
       past: s.past.slice(0, -1),
       present: prev,
@@ -67,7 +67,8 @@ export function useUndoable<T>(
   const redo = useCallback(() => {
     const s = ref.current;
     if (!s.future.length) return;
-    const [next, ...rest] = s.future;
+    const next = s.future[0]!; // future is non-empty (guarded above)
+    const rest = s.future.slice(1);
     ref.current = { past: [...s.past, s.present], present: next, future: rest };
     force();
     onApplyRef.current?.(next);