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/linkify.tsx

@@ -62,10 +62,14 @@ export function linkify(text: string): ReactNode[] {
       const raw = m[1];
       out.push(link(/^https?:\/\//i.test(raw) ? raw : `https://${raw}`, raw));
     } else {
-      // "<platform>: @handle" — keep the "<platform>: " prefix as text, link only the handle.
-      const handle = m[3];
+      // Reached only when the MENTION alternative matched, so its groups are present: m[2] is the
+      // platform (always one of PLATFORMS' own keys — the pattern is built from them) and m[3] the
+      // handle. "<platform>: @handle" — keep the "<platform>: " prefix as text, link only the handle.
+      const platform = m[2]!;
+      const handle = m[3]!;
+      const base = PLATFORMS[platform.toLowerCase()]!;
       out.push(<Fragment key={key++}>{m[0].slice(0, m[0].length - handle.length - 1)}</Fragment>);
-      out.push(link(PLATFORMS[m[2].toLowerCase()] + handle, "@" + handle));
+      out.push(link(base + handle, "@" + handle));
     }
     last = m.index + m[0].length;
   }