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++}