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/views/SubscriptionsView.vue

@@ -21,7 +21,7 @@
       </v-tab>
     </v-tabs>
 
-    <v-window v-model="tab" class="mt-4">
+    <v-window v-model="tab" class="mt-4 fc-subs-window">
       <v-window-item value="subscriptions">
         <SubscriptionsTab />
       </v-window-item>
@@ -52,8 +52,22 @@ const { tab } = useTabQuery(VALID_TABS, 'subscriptions')
 .fc-subs-shell {
   max-width: 1600px;
   margin-inline: auto;
+  /* Fixed-height hub: the tabs (and each tab's sticky control bar) stay
+     put while ONLY the tab content scrolls — previously the whole view
+     scrolled instead of just the subscription list (operator-flagged
+     2026-05-28). 64px = the TopNav height (AppShell .fc-content pad-top). */
+  height: calc(100vh - 64px);
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
 }
 .fc-subs-tabs {
+  flex: 0 0 auto;
   border-bottom: 1px solid rgb(var(--v-theme-on-surface-variant) / 0.18);
 }
+.fc-subs-window {
+  flex: 1 1 auto;
+  min-height: 0;
+  overflow-y: auto;
+}
 </style>