feat(series): Add-to-series control + Series browse view + nav (FC-6.2)

Completes FC-6.2 with the UI.

- PostSeriesMenu: a "Series ▾" control on each post card — "New series from
  this post" (promote → navigates to manage) and "Add to existing series…"
  (dialog with a browsable picker loaded from GET /api/series, client-side
  filtered — avoids the empty-autocomplete #712 trap).
- SeriesView (/series): a top-level Series browse grid — cover, name, artist,
  chapter/page counts, gap badge; sort recent|name|size; cards → manage/read.
  meta.title adds it to the nav automatically (peer of Posts).
- seriesBrowse store for the list.

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

frontend/src/components/posts/PostCard.vue

@@ -15,6 +15,7 @@
         · {{ totalImages }} image{{ totalImages === 1 ? '' : 's' }}
       </span>
       <v-spacer />
+      <PostSeriesMenu :post="post" />
       <v-btn
         v-if="post.post_url"
         :href="post.post_url" target="_blank" rel="noopener"
@@ -101,6 +102,7 @@ import { useModalStore } from '../../stores/modal.js'
 import { usePostsStore } from '../../stores/posts.js'
 import { toPlainText } from '../../utils/htmlSanitize.js'
 import PostEmptyThumbs from './PostEmptyThumbs.vue'
+import PostSeriesMenu from './PostSeriesMenu.vue'
 
 const props = defineProps({
   post: { type: Object, required: true },