refactor(dry-F5): useTabQuery composable for ?tab= query-param routing

ArtistView and SubscriptionsView both two-way-bound a tab ref with the
?tab= query param via identical tab->URL and URL->tab watchers. Extracted
useTabQuery(validTabs, defaultTab) — defaultTab accepts a string or a
function (ArtistView's default is postCount-dependent), and resolve() is
exposed so ArtistView can re-apply it after the artist loads. Both views
drop their now-orphaned ref/useRouter imports.

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

frontend/src/views/SubscriptionsView.vue

@@ -36,32 +36,14 @@
 </template>
 
 <script setup>
-import { ref, watch } from 'vue'
-import { useRoute, useRouter } from 'vue-router'
-
 import SubscriptionsTab from '../components/subscriptions/SubscriptionsTab.vue'
 import DownloadsTab from '../components/subscriptions/DownloadsTab.vue'
 import SettingsTab from '../components/subscriptions/SettingsTab.vue'
+import { useTabQuery } from '../composables/useTabQuery.js'
 
 const VALID_TABS = ['subscriptions', 'downloads', 'settings']
 
-const route = useRoute()
-const router = useRouter()
-
-const tab = ref(
-  VALID_TABS.includes(route.query.tab) ? route.query.tab : 'subscriptions',
-)
-
-watch(tab, (t) => {
-  if (route.query.tab === t) return
-  router.replace({ query: { ...route.query, tab: t } })
-})
-
-watch(() => route.query.tab, (q) => {
-  if (q && VALID_TABS.includes(q) && tab.value !== q) {
-    tab.value = q
-  }
-})
+const { tab } = useTabQuery(VALID_TABS, 'subscriptions')
 </script>
 
 <style scoped>