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/descriptionLinks.tsx

@@ -14,7 +14,8 @@ const INLINE_RE =
 
 function tsToSeconds(ts: string): number {
   const parts = ts.split(":").map((n) => parseInt(n, 10));
-  return parts.length === 3 ? parts[0] * 3600 + parts[1] * 60 + parts[2] : parts[0] * 60 + parts[1];
+  const [a = 0, b = 0, c = 0] = parts; // hh:mm:ss or mm:ss — absent segments default to 0
+  return parts.length === 3 ? a * 3600 + b * 60 + c : a * 60 + b;
 }
 
 // Parse a YouTube `t`/`start` param: "90", "90s", or "1h2m3s".
@@ -44,7 +45,7 @@ function parseYouTube(url: string): { id: string; start: number | null } | null
     if (u.pathname === "/watch") id = u.searchParams.get("v");
     else {
       const m = u.pathname.match(/^\/(?:shorts|embed|live)\/([^/?#]+)/);
-      if (m) id = m[1];
+      if (m) id = m[1] ?? null;
     }
   }
   if (!id) return null;
@@ -128,7 +129,9 @@ export function renderDescription(
           </a>,
         );
       } else {
-        const ts = m[3];
+        // Reached only when neither m[1] (email) nor m[2] (hashtag) matched, so the third
+        // alternative — the timestamp group — is the one that fired.
+        const ts = m[3]!;
         out.push(
           <button
             key={key++}

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"],

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;
   }

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]!;
 }

frontend/src/lib/useUndoable.ts

@@ -54,7 +54,7 @@ export function useUndoable<T>(
   const undo = useCallback(() => {
     const s = ref.current;
     if (!s.past.length) return;
-    const prev = s.past[s.past.length - 1];
+    const prev = s.past[s.past.length - 1]!; // past is non-empty (guarded above)
     ref.current = {
       past: s.past.slice(0, -1),
       present: prev,
@@ -67,7 +67,8 @@ export function useUndoable<T>(
   const redo = useCallback(() => {
     const s = ref.current;
     if (!s.future.length) return;
-    const [next, ...rest] = s.future;
+    const next = s.future[0]!; // future is non-empty (guarded above)
+    const rest = s.future.slice(1);
     ref.current = { past: [...s.past, s.present], present: next, future: rest };
     force();
     onApplyRef.current?.(next);