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/format.ts

@@ -33,7 +33,12 @@ export function formatSpeed(bps: number | null | undefined): string {
  * MUST stay sorted ascending, with the first row doubling as the "just now" threshold: sorting it
  * largest-first — the natural instinct — would make every timestamp under a year read "just now".
  * Module-level because it is constant, and exported so the tests can hold that invariant. */
-export const RELATIVE_UNITS: readonly [number, Intl.RelativeTimeFormatUnit][] = [
+// A non-empty tuple type (first element + rest) so `RELATIVE_UNITS[0]` is known-present under
+// noUncheckedIndexedAccess — the "sub-minute" branch relies on that first entry always existing.
+export const RELATIVE_UNITS: readonly [
+  [number, Intl.RelativeTimeFormatUnit],
+  ...[number, Intl.RelativeTimeFormatUnit][],
+] = [
   [60, "minute"],
   [3600, "hour"],
   [86400, "day"],