fix(subscriptions): fixed-height hub so only the tab content scrolls

The whole view scrolled instead of just the subscription list. Made the
hub a viewport-height flex column (tabs stay fixed) with the v-window as
the single internal scroll container; the per-tab sticky control bars now
pin to the window top (top:48px -> 0). Operator-flagged 2026-05-28.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/subscriptions/DownloadsTab.vue

@@ -334,7 +334,7 @@ async function openDetail(id) {
    background so rows don't bleed through. */
 .fc-dl__sticky {
   position: sticky;
-  top: 48px;
+  top: 0;
   z-index: 3;
   background: rgb(var(--v-theme-background));
   padding: 8px 0 10px;