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/router.js

@@ -4,6 +4,7 @@ import GalleryView from './views/GalleryView.vue'
 import ShowcaseView from './views/ShowcaseView.vue'
 import TagsView from './views/TagsView.vue'
 import ArtistView from './views/ArtistView.vue'
+import SeriesView from './views/SeriesView.vue'
 import SeriesManageView from './views/SeriesManageView.vue'
 import SeriesReaderView from './views/SeriesReaderView.vue'
 import SubscriptionsView from './views/SubscriptionsView.vue'
@@ -25,7 +26,9 @@ const routes = [
   { path: '/tags',     name: 'tags',     component: TagsView,     meta: { title: 'Tags'     } },
   // Artist detail — no meta.title (reached by clicking an artist, not nav).
   { path: '/artist/:slug', name: 'artist', component: ArtistView },
-  // Series management — no meta.title (reached from a series tag card).
+  // Series browse — a nav entry (meta.title). Peer of Posts.
+  { path: '/series', name: 'series', component: SeriesView, meta: { title: 'Series' } },
+  // Series management — no meta.title (reached from a series card/tag).
   { path: '/series/:tagId', name: 'series-manage', component: SeriesManageView },
   // Series reader — immersive (no top nav, no meta.title).
   { path: '/series/:tagId/read', name: 'series-read', component: SeriesReaderView, meta: { immersive: true } },