feat(nav): consolidate Posts/Artists/Tags into a Browse hub

Posts, Artists, and Tags are the three 'browse the library by an axis'
surfaces; Subscriptions stays purely management (operator-asked 2026-06-09).
New BrowseView renders them as tabs (?tab=posts|artists|tags); only the active
tab mounts. The old standalone paths become redirects into the matching tab,
preserving deep-link query (/posts?post_id=N → /browse?tab=posts&post_id=N) and
keeping the route names so existing { name: 'posts'|'artists'|'tags' } links and
path pushes still resolve. Nav now reads Showcase · Gallery · Browse · Series ·
Subscriptions, with Settings pinned right.

Test: /browse resolves; /tags and /artists redirect into their tabs; a posts
deep link survives the redirect.

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

frontend/src/views/BrowseView.vue

@@ -0,0 +1,47 @@
+<template>
+  <div class="fc-browse">
+    <v-container fluid class="pt-2 pb-0">
+      <v-tabs v-model="tab" density="compact">
+        <v-tab value="posts">Posts</v-tab>
+        <v-tab value="artists">Artists</v-tab>
+        <v-tab value="tags">Tags</v-tab>
+      </v-tabs>
+    </v-container>
+
+    <!-- Each tab's view is self-contained (its own container + state). Only the
+         active one is mounted; switching tabs swaps it. The Posts tab still
+         reads its deep-link (post_id/artist_id/platform) from route.query, so
+         /browse?tab=posts&post_id=N works exactly like the old /posts deep link.
+         (FC nav consolidation, operator-asked 2026-06-09: Posts/Artists/Tags are
+         the three "browse the library by an axis" surfaces.) -->
+    <component :is="activeComponent" :key="tab" />
+  </div>
+</template>
+
+<script setup>
+import { computed } from 'vue'
+import { useRoute, useRouter } from 'vue-router'
+
+import ArtistsView from './ArtistsView.vue'
+import PostsView from './PostsView.vue'
+import TagsView from './TagsView.vue'
+
+const TABS = { posts: PostsView, artists: ArtistsView, tags: TagsView }
+
+const route = useRoute()
+const router = useRouter()
+
+const tab = computed({
+  get() {
+    const t = route.query.tab
+    return typeof t === 'string' && t in TABS ? t : 'posts'
+  },
+  set(value) {
+    // Switching tabs starts a clean tab — a posts deep link (post_id, etc.)
+    // belongs only to the Posts tab and shouldn't bleed into Artists/Tags.
+    router.replace({ name: 'browse', query: { tab: value } })
+  },
+})
+
+const activeComponent = computed(() => TABS[tab.value])
+</script>