feat(series): chapter-aware manage view + reader — frontend (FC-6.1)

Completes FC-6.1: the series management UI now works in chapters.

- SeriesManageView: chapters as cards (inline-rename, stated-page range inputs,
  move up/down, merge-into-previous, delete, pick-as-add-target), pages
  drag-reorder WITHIN a chapter, a "gap: N-M missing" badge between chapters
  with a stated-page hole, and Add chapter / Add placeholder. The picker adds
  the selection into the targeted chapter.
- seriesManage store: chapter CRUD + reorderChapters/moveChapter/mergeChapter/
  reorderPages actions; consumes chapters[]/gaps[]; addSelected targets a chapter.
- Reader: page_number is now within-chapter, so anchors switched to a global
  `seq` (reading-order position) — fixes scroll/jump/active collisions across
  chapters — plus chapter-title dividers at each chapter boundary.
- Updated seriesManage.spec to the chaptered store shape.

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

frontend/src/views/SeriesReaderView.vue

@@ -20,9 +20,9 @@
           :value="currentPage" @change="jumpTo($event.target.value)"
         >
           <option
-            v-for="p in store.pages" :key="p.page_number"
-            :value="p.page_number"
-          >Page {{ p.page_number }}</option>
+            v-for="p in store.pages" :key="p.seq"
+            :value="p.seq"
+          >Page {{ p.seq }}</option>
         </select>
         <v-btn
           icon="mdi-menu" variant="text" size="small"
@@ -43,23 +43,27 @@
         <div
           v-for="p in store.pages" :key="p.image_id"
           class="fc-reader__thumb"
-          :class="{ active: p.page_number === currentPage }"
-          @click="jumpTo(p.page_number, true)"
+          :class="{ active: p.seq === currentPage }"
+          @click="jumpTo(p.seq, true)"
         >
           <img :src="p.thumbnail_url" alt="" loading="lazy" />
-          <span class="fc-reader__thumbnum">{{ p.page_number }}</span>
+          <span class="fc-reader__thumbnum">{{ p.seq }}</span>
         </div>
       </div>
     </aside>
 
     <div ref="scrollEl" class="fc-reader__content" @scroll="onScroll">
-      <div
-        v-for="p in store.pages" :key="p.image_id"
-        class="fc-reader__page" :id="'fc-page-' + p.page_number"
-        :data-page="p.page_number"
-      >
-        <img :src="p.image_url" :alt="'Page ' + p.page_number" loading="lazy" />
-      </div>
+      <template v-for="p in store.pages" :key="p.image_id">
+        <div v-if="p.isChapterStart" class="fc-reader__chdiv">
+          {{ p.chapterLabel }}
+        </div>
+        <div
+          class="fc-reader__page" :id="'fc-page-' + p.seq"
+          :data-page="p.seq"
+        >
+          <img :src="p.image_url" :alt="'Page ' + p.seq" loading="lazy" />
+        </div>
+      </template>
       <div v-if="store.error" class="fc-reader__empty">
         {{ store.error }}
         <v-btn variant="text" color="accent" @click="goBack">Back</v-btn>
@@ -115,9 +119,9 @@ function pageMetrics() {
   const root = scrollEl.value
   if (!root) return []
   return store.pages.map(p => {
-    const el = root.querySelector('#fc-page-' + p.page_number)
+    const el = root.querySelector('#fc-page-' + p.seq)
     return {
-      page_number: p.page_number,
+      page_number: p.seq,
       top: el ? el.offsetTop : 0,
       height: el ? el.offsetHeight : 0
     }
@@ -280,6 +284,13 @@ onUnmounted(() => {
   display: flex; flex-direction: column; align-items: center;
   gap: 2px; padding: 0.25rem;
 }
+.fc-reader__chdiv {
+  width: 100%; max-width: 1200px; margin: 0.5rem auto 0;
+  padding: 0.4rem 0.75rem;
+  font-family: 'Fraunces', Georgia, serif; font-size: 0.95rem;
+  color: rgb(var(--v-theme-on-surface-variant));
+  border-top: 1px solid rgb(var(--v-theme-surface-light));
+}
 .fc-reader__page { width: 100%; display: flex; justify-content: center; }
 .fc-reader__page img {
   max-width: min(100%, 1200px); height: auto; display: block;