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 },

frontend/src/components/posts/PostSeriesMenu.vue

@@ -0,0 +1,123 @@
+<template>
+  <span class="fc-post-series">
+    <v-btn
+      size="x-small" variant="text" prepend-icon="mdi-book-plus-outline"
+      append-icon="mdi-menu-down"
+      :aria-label="`Add post ${post.id} to a series`"
+      @click.stop="menuOpen = !menuOpen"
+    >Series</v-btn>
+    <v-menu v-model="menuOpen" activator="parent" :open-on-click="false">
+      <v-list density="compact">
+        <v-list-item :disabled="busy" @click="onPromote">
+          <template #prepend>
+            <v-icon size="small">mdi-book-plus</v-icon>
+          </template>
+          <v-list-item-title>New series from this post</v-list-item-title>
+        </v-list-item>
+        <v-list-item @click="openPicker">
+          <template #prepend>
+            <v-icon size="small">mdi-playlist-plus</v-icon>
+          </template>
+          <v-list-item-title>Add to existing series…</v-list-item-title>
+        </v-list-item>
+      </v-list>
+    </v-menu>
+
+    <v-dialog v-model="pickerOpen" max-width="460">
+      <v-card>
+        <v-card-title>Add to series</v-card-title>
+        <v-card-text>
+          <p class="text-caption mb-2">
+            Appends this post as the next chapter of the chosen series.
+          </p>
+          <v-autocomplete
+            v-model="picked"
+            :items="hits"
+            :item-title="(s) => s.name"
+            :item-value="(s) => s.id"
+            :loading="searching"
+            label="Series" density="compact" autofocus clearable
+          />
+        </v-card-text>
+        <v-card-actions>
+          <v-spacer />
+          <v-btn @click="pickerOpen = false">Cancel</v-btn>
+          <v-btn
+            color="primary" rounded="pill"
+            :disabled="picked == null || busy" @click="onAddExisting"
+          >Add</v-btn>
+        </v-card-actions>
+      </v-card>
+    </v-dialog>
+  </span>
+</template>
+
+<script setup>
+import { ref } from 'vue'
+import { useRouter } from 'vue-router'
+import { useApi } from '../../composables/useApi.js'
+import { toast } from '../../utils/toast.js'
+
+const props = defineProps({ post: { type: Object, required: true } })
+
+const api = useApi()
+const router = useRouter()
+
+const menuOpen = ref(false)
+const busy = ref(false)
+const pickerOpen = ref(false)
+const picked = ref(null)
+const hits = ref([])
+const searching = ref(false)
+
+async function onPromote() {
+  menuOpen.value = false
+  busy.value = true
+  try {
+    const out = await api.post('/api/series/from-post', {
+      body: { post_id: props.post.id }
+    })
+    toast({ text: `Series created: ${out.name}`, type: 'success' })
+    router.push({ name: 'series-manage', params: { tagId: out.series_tag_id } })
+  } catch (e) {
+    toast({ text: `Could not create series: ${e.message}`, type: 'error' })
+  } finally {
+    busy.value = false
+  }
+}
+
+async function openPicker() {
+  menuOpen.value = false
+  picked.value = null
+  hits.value = []
+  pickerOpen.value = true
+  // Load the full series list so the picker is browsable (the tag autocomplete
+  // returns nothing for an empty query — the #712 trap). v-autocomplete filters
+  // client-side by name as the operator types.
+  searching.value = true
+  try {
+    const body = await api.get('/api/series', { params: { sort: 'name' } })
+    hits.value = body.series || []
+  } catch (e) {
+    toast({ text: `Could not load series: ${e.message}`, type: 'error' })
+  } finally {
+    searching.value = false
+  }
+}
+
+async function onAddExisting() {
+  if (picked.value == null) return
+  busy.value = true
+  try {
+    await api.post(`/api/series/${picked.value}/add-post`, {
+      body: { post_id: props.post.id }
+    })
+    pickerOpen.value = false
+    toast({ text: 'Added to series', type: 'success' })
+  } catch (e) {
+    toast({ text: `Could not add to series: ${e.message}`, type: 'error' })
+  } finally {
+    busy.value = false
+  }
+}
+</script>