feat: upload flow revamp (0.20.0)

- `/upload` is now manga-only with optional N initial chapters
  staged inline.
- Additional chapters from a new `/manga/[id]/upload-chapter` route,
  reached via an "Upload chapter" button on the manga page.
- New `ChapterPagesEditor` component: thumbnails next to each row,
  click-to-preview-modal, drag-drop + reorder.
- Pages renamed to `page-NNN.<ext>` before multipart submission;
  original filenames shown as dimmed reference text during upload
  and dropped on submit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.19.2"
+version = "0.20.0"
 edition = "2021"
 
 [lib]