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/stores/seriesBrowse.js

@@ -0,0 +1,31 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+import { useApi } from '../composables/useApi.js'
+import { useAsyncAction } from '../composables/useAsyncAction.js'
+
+// Backs the Series browse view (FC-6.2). list_series returns every series as a
+// card; homelab scale is small enough to load the whole list and sort/filter
+// server-side.
+export const useSeriesBrowseStore = defineStore('seriesBrowse', () => {
+  const api = useApi()
+  const series = ref([])
+  const sort = ref('recent')
+  const artistId = ref(null)
+  const { loading, error, run } = useAsyncAction({ errorAs: 'message' })
+
+  async function load() {
+    await run(async () => {
+      const params = { sort: sort.value }
+      if (artistId.value != null) params.artist_id = artistId.value
+      const body = await api.get('/api/series', { params })
+      series.value = body.series || []
+    })
+  }
+
+  function setSort(s) {
+    sort.value = s
+    return load()
+  }
+
+  return { series, sort, artistId, loading, error, load, setSort }
+})