feat: admin UX overhaul — status filters, retry/cancel, mobile cards, i18n, shelf pre-populate

- Admin layout: SVG icons, active highlight, divider between nav sections
- Scrape page: status filter pills with counts, text + status combined search
- Audio page: status filter pills, cancel jobs, retry failed jobs, mobile cards for cache tab
- Translation page: status filter pills (incl. cancelled), cancel + retry jobs, mobile cancel/retry cards, i18n for all labels
- AI Jobs page: fix concurrent cancel (Set instead of single slot), per-job cancel errors inline, full mobile card layout, i18n title/heading
- Text-gen page: tagline editable input + copy, warnings copy, i18n title/heading
- Book page: chapter cover Save button, audio monitor link, currentShelf pre-populated from server
- pocketbase.ts: add getBookShelf(), shelf field on UserLibraryEntry
- New API route: POST /api/admin/translation/bulk (proxy for translation retry)
- i18n: 15 new admin_translation_*, admin_ai_jobs_*, admin_text_gen_* keys across all 5 locales

backend/internal/bookstore/bookstore.go

@@ -171,6 +171,20 @@ type AIJobStore interface {
 	ListAIJobs(ctx context.Context) ([]domain.AIJob, error)
 }
 
+// ChapterImageStore covers per-chapter illustration images stored in MinIO.
+// The backend admin writes them; the backend serves them.
+type ChapterImageStore interface {
+	// PutChapterImage stores a raw image for chapter n of slug in MinIO.
+	PutChapterImage(ctx context.Context, slug string, n int, data []byte, contentType string) error
+
+	// GetChapterImage retrieves the image for chapter n of slug.
+	// Returns (nil, "", false, nil) when no image exists.
+	GetChapterImage(ctx context.Context, slug string, n int) ([]byte, string, bool, error)
+
+	// ChapterImageExists returns true when an image is stored for slug/n.
+	ChapterImageExists(ctx context.Context, slug string, n int) bool
+}
+
 // TranslationStore covers machine-translated chapter storage in MinIO.
 // The runner writes translations; the backend reads them.
 type TranslationStore interface {