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/ConfigPanel.tsx

@@ -81,6 +81,7 @@ export default function ConfigPanel() {
     try {
       for (const key of dirtyKeys) {
         const it = byKey[key];
+        if (!it) continue; // dirtyKeys are all config-item keys; skip if somehow not found
         const raw = draft[key] ?? "";
         if (it.secret) {
           if (secretReset[key] && !raw.trim()) await api.resetConfig(key);
@@ -170,14 +171,14 @@ export default function ConfigPanel() {
       <PageToolbar>
         <div className="px-4 pt-3 max-w-3xl w-full mx-auto">
           <p className="text-xs text-muted mb-3 leading-relaxed">{t("config.intro")}</p>
-          <Tabs tabs={groupTabs} active={activeGroup} onChange={setTab} />
+          <Tabs tabs={groupTabs} active={activeGroup ?? ""} onChange={setTab} />
         </div>
       </PageToolbar>
       <div className="px-4 pb-24 pt-3 max-w-3xl w-full mx-auto">
         {activeGroup && (
           <div className="glass rounded-2xl p-4 mb-4">
             <div className="divide-y divide-border/60">
-              {data.groups[activeGroup].map((item) => (
+              {(data.groups[activeGroup] ?? []).map((item) => (
                 <Field
                   key={item.key}
                   item={item}