fix(ui): mobile pass 2 — Posts & Subscriptions filter bars

PostsFilterBar didn't wrap and its artist/platform fields had inline
min-widths (240/180px) a media query can't override → horizontal overflow on
phones. Moved widths to classes, added flex-wrap, and <600px each field takes
a full-width row. SubscriptionsTab's status/search inline max-widths likewise
moved to classes; <600px they go full-width and the v-spacer is dropped so the
search isn't shoved around.

Verified as already-fine (sweep false positives, no change): PostCard (default
body is a stacked column; only goes row at container >=800px), SeriesReaderView
(already has a <=768px block: nav drawer 150px, quick-nav stacks). The
subscriptions v-data-table scrolls horizontally within its own wrapper, so it
doesn't widen the page — a true mobile card layout is a larger follow-up if
wanted.

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

frontend/src/components/subscriptions/SubscriptionsTab.vue

@@ -26,14 +26,14 @@
         :items="STATUS_OPTIONS"
         :disabled="needsAttention"
         density="compact" variant="outlined" hide-details
-        style="max-width: 180px"
+        class="fc-subs__status"
       />
       <v-text-field
         v-model="search"
         density="compact" variant="outlined" hide-details clearable
         prepend-inner-icon="mdi-magnify"
         placeholder="Search subscriptions"
-        style="max-width: 320px"
+        class="fc-subs__search"
       />
     </div>
 
@@ -568,6 +568,14 @@ async function bulkDelete() {
   padding: 8px 0 1rem;
 }
 .fc-subs__bar .v-chip { cursor: pointer; }
+.fc-subs__status { max-width: 180px; }
+.fc-subs__search { max-width: 320px; flex: 1 1 200px; }
+/* Phones: status + search each take a full-width row; drop the spacer that
+   would otherwise eat a row pushing them around. */
+@media (max-width: 600px) {
+  .fc-subs__status, .fc-subs__search { max-width: none; flex-basis: 100%; }
+  .fc-subs__bar :deep(.v-spacer) { display: none; }
+}
 .fc-subs__loading, .fc-subs__empty {
   display: flex; justify-content: center; padding: 2rem;
   color: rgb(var(--v-theme-on-surface-variant));