feat(series): manage-view redesign — big pages, editable Part #, slide-over picker (FC-6.4)

Operator feedback: thumbnails too small to judge order, no obvious way to mark
'this installment is Part 2', and the permanent two-pane picker was busy and
competed with the ordering work.

- Full-width parts, each a card with a big page grid (150px, contain so whole
  pages are visible) and drag-to-reorder; positional page number as a badge.
- Editable Part # (hero field) backed by new series_chapter.stated_part —
  separate from the auto-managed chapter_number, mirroring the page_number vs
  stated_page split so reorder/delete renumbering can't wipe a hand-set part.
  Missing-Part hints when consecutive parts' stated_part jump >1.
- Each part labels its source post (derived from pages' primary_post_id) and
  shows the printed-page range with clear labels.
- Picker demoted to an on-demand right slide-over ('Add pages') with a target-
  part selector; part actions (move/merge/delete) collapsed into an overflow ⋮.

alembic 0042 adds series_chapter.stated_part (nullable int).

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

frontend/test/seriesManage.spec.js

@@ -75,6 +75,33 @@ describe('seriesManage', () => {
     expect(r.body).toEqual({ chapter_ids: [2, 1, 3] })
   })
 
+  it('setChapterPart patches stated_part on the chapter', async () => {
+    const s = useSeriesManageStore()
+    s.tagId = 7
+    const calls = []
+    stubFetch((url, init) => {
+      calls.push({ url, method: init.method, body: init.body ? JSON.parse(init.body) : null })
+      if (init.method === 'PATCH') return { status: 200, body: { ok: true } }
+      return { status: 200, body: SERIES_BODY }
+    })
+    await s.setChapterPart(1, 2)
+    const p = calls.find(c => c.method === 'PATCH')
+    expect(p.url).toContain('/api/series/7/chapters/1')
+    expect(p.body).toEqual({ stated_part: 2 })
+  })
+
+  it('load surfaces part_gaps and partGapAfter looks them up', async () => {
+    const s = useSeriesManageStore()
+    stubFetch(() => ({
+      status: 200,
+      body: { ...SERIES_BODY, part_gaps: [{ after_chapter_id: 1, start: 2, end: 2 }] }
+    }))
+    await s.load(7)
+    expect(s.partGaps).toHaveLength(1)
+    expect(s.partGapAfter(1)).toEqual({ after_chapter_id: 1, start: 2, end: 2 })
+    expect(s.partGapAfter(99)).toBeNull()
+  })
+
   it('addSelected posts selection + target chapter then clears', async () => {
     const s = useSeriesManageStore()
     s.tagId = 7