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/VideoEditor.tsx

@@ -87,7 +87,12 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
     if (!v) return;
     const d = v.duration && isFinite(v.duration) ? v.duration : srcDur;
     setDuration(d);
-    setSegments((s) => (s.length === 1 && s[0].end <= 0 ? [{ id: 0, start: 0, end: d, keep: true }] : s));
+    // Reconcile the still-pristine full-length segment to the REAL decoded duration. The initial
+    // segment ends at the (integer, often-rounded) metadata `srcDur`; once the media loads we know
+    // the true length (e.g. 213.44 vs 213). Without this the untouched last segment stays short, so
+    // the no-op guard (isFullSingle: end >= duration) reads false and enables a "Create" that files a
+    // clip of an unmodified video and drops its tail. Only touches the untouched segment (end===srcDur).
+    setSegments((s) => (s.length === 1 && s[0].end === srcDur ? [{ ...s[0], end: d }] : s));
   };
   const onTimeUpdate = () => {
     const v = videoRef.current;
@@ -242,7 +247,7 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
     segments.length === 1 && kept.length === 1 && kept[0].start <= 0.01 && kept[0].end >= duration - 0.01;
   const valid = kept.length >= 1 && keptDur >= MIN_SEG && (cropOn || !isFullSingle);
   const willJoin = output === "join" && kept.length > 1;
-  const reencode = cropOn || (willJoin ? accurate : accurate);
+  const reencode = cropOn || accurate;
 
   const cropPx = (): { x: number; y: number; w: number; h: number } | undefined => {
     const v = videoRef.current;