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/GlassTuner.tsx

@@ -339,7 +339,12 @@ export default function GlassTuner() {
               {tab === "glass" && (
                 <div className="flex flex-col gap-2.5">
                   {SLIDERS.map((s) => (
-                    <Ctl key={s.k} s={s} value={vars[s.k]} onChange={(v) => setVar(s.k, v)} />
+                    <Ctl
+                      key={s.k}
+                      s={s}
+                      value={vars[s.k] ?? s.def}
+                      onChange={(v) => setVar(s.k, v)}
+                    />
                   ))}
                 </div>
               )}