fix(ui): subscriptions table → card layout on mobile

The subscriptions v-data-table (select + expand + 6 cols + a nested 8-col
sources sub-table) horizontally-scrolled on phones. Set mobile-breakpoint=600
so Vuetify stacks each subscription row into a label:value card below 600px;
the custom item slots (platform chips, health dot, action buttons) render as
card rows. The expanded sources detail reclaims its desktop indent on mobile
and keeps its own horizontal scroll for the wide source columns.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/subscriptions/SubscriptionsTab.vue

@@ -86,6 +86,7 @@
         :items-per-page-options="ITEMS_PER_PAGE_OPTIONS"
         density="comfortable"
         hover show-select show-expand
+        :mobile-breakpoint="600"
         @click:row="onRowClick"
       >
         <template #item.name="{ item }">
@@ -575,6 +576,10 @@ async function bulkDelete() {
 @media (max-width: 600px) {
   .fc-subs__status, .fc-subs__search { max-width: none; flex-basis: 100%; }
   .fc-subs__bar :deep(.v-spacer) { display: none; }
+  /* The table renders as stacked cards (mobile-breakpoint); reclaim the
+     desktop indent on the expanded sources detail (it keeps its own
+     horizontal scroll for the wide source columns). */
+  .fc-subs__sources-cell { padding-left: 0.5rem !important; }
 }
 .fc-subs__loading, .fc-subs__empty {
   display: flex; justify-content: center; padding: 2rem;