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>

frontend/src/routes/manga/[id]/+page.svelte

@@ -14,6 +14,7 @@
     import AddToCollectionModal from '$lib/components/AddToCollectionModal.svelte';
     import Plus from '@lucide/svelte/icons/plus';
     import FolderPlus from '@lucide/svelte/icons/folder-plus';
+    import UploadCloud from '@lucide/svelte/icons/upload-cloud';
 
     let { data } = $props();
     const manga = $derived(data.manga);
@@ -323,6 +324,14 @@
                         <FolderPlus size={16} aria-hidden="true" />
                         <span>Add to collection</span>
                     </button>
+                    <a
+                        class="action"
+                        href="/manga/{manga.id}/upload-chapter"
+                        data-testid="upload-chapter-link"
+                    >
+                        <UploadCloud size={16} aria-hidden="true" />
+                        <span>Upload chapter</span>
+                    </a>
                 </div>
             {:else}
                 <a class="action" href="/login" data-testid="bookmark-signin">