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/modules.ts

@@ -128,6 +128,7 @@ export function stepModule(me: Me, current: Page, dir: 1 | -1): Page {
   const order = moduleOrder(me);
   if (order.length === 0) return current;
   const i = order.indexOf(current);
-  if (i === -1) return dir === 1 ? order[0] : order[order.length - 1];
-  return order[(i + dir + order.length) % order.length];
+  // order is non-empty (guarded above) and every index below is in-bounds, so none are undefined.
+  if (i === -1) return dir === 1 ? order[0]! : order[order.length - 1]!;
+  return order[(i + dir + order.length) % order.length]!;
 }