fix(downloads-ui): editor tail no-op + persisted-tab fallback (+ dead ternary)

Found during the Phase-2 Downloads-frontend review:
- VideoEditor: onLoaded now reconciles the still-pristine full-length segment to the
  REAL decoded duration (its guard `end<=0` was dead since the metadata srcDur is
  always >0). Without this the untouched last segment kept the rounded metadata
  length, so isFullSingle read false and "Create" was enabled on an unmodified
  video — producing a bogus edit that dropped the tail.
- DownloadCenter: fall back to the "queue" tab when a persisted tab (e.g. admin-only
  "system") isn't in the current set, instead of rendering a blank page.
- VideoEditor: reencode = cropOn || accurate (was a no-op `willJoin ? accurate : accurate`).

tsc clean. GUI-affecting → pending an E2E visual test (needs an authed localdev
session) before user UAT. Cross-cutting UI-consistency cleanup deferred to Phase 3.

frontend/src/components/DownloadCenter.tsx

@@ -1,4 +1,4 @@
-import { lazy, Suspense, useMemo, useState } from "react";
+import { lazy, Suspense, useEffect, useMemo, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import {
@@ -586,6 +586,12 @@ export default function DownloadCenter({ me }: { me: Me }) {
     { id: "shared", label: t("downloads.tabs.shared") },
     ...(me.role === "admin" ? [{ id: "system", label: t("downloads.tabs.system") }] : []),
   ];
+  // A persisted tab can point at one no longer available (e.g. "system" restored for a now-non-admin
+  // account) — that would render a blank page with no active tab. Fall back to the default.
+  useEffect(() => {
+    if (!tabs.some((tb) => tb.id === tab)) setTab("queue");
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [tab, me.role]);
 
   const saveBtn = (job: DownloadJob) => (
     <a