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/lib/api/chapters.ts

@@ -55,3 +55,66 @@ export async function getChapterPages(
     );
     return r.pages;
 }
+
+export type NewChapter = {
+    number: number;
+    title?: string | null;
+};
+
+/**
+ * `POST /api/v1/mangas/:id/chapters` is multipart: a `metadata` part
+ * (JSON) plus one or more ordered `page` parts. Each page file is
+ * renamed to `page-NNN.<ext>` before submission so the user's
+ * original filenames (often personally-identifying or just messy:
+ * `IMG_2837.HEIC`, `~/scans/full chapter pack/`) don't end up in
+ * request bodies or server logs. The bytes are unchanged — the
+ * backend still sniffs the MIME from magic bytes and stores under
+ * its own `{nnnn}.{ext}` scheme.
+ */
+export async function createChapter(
+    mangaId: string,
+    metadata: NewChapter,
+    pages: File[]
+): Promise<Chapter> {
+    const form = new FormData();
+    form.append(
+        'metadata',
+        new Blob([JSON.stringify(metadata)], { type: 'application/json' })
+    );
+    pages.forEach((file, i) => {
+        const ext = extensionFor(file);
+        const renamed = new File(
+            [file],
+            `page-${String(i + 1).padStart(3, '0')}${ext}`,
+            { type: file.type }
+        );
+        form.append('page', renamed);
+    });
+    return request<Chapter>(
+        `/v1/mangas/${encodeURIComponent(mangaId)}/chapters`,
+        { method: 'POST', body: form }
+    );
+}
+
+/**
+ * Pick a sensible extension for the renamed multipart part. Prefer
+ * the original filename's extension when present (jpg/jpeg/png/webp/
+ * gif/avif), otherwise derive from the MIME type. Falls back to an
+ * empty string so the renamed file is just `page-001` — the
+ * server sniffs bytes anyway.
+ */
+function extensionFor(file: File): string {
+    const dot = file.name.lastIndexOf('.');
+    if (dot > 0) {
+        const ext = file.name.slice(dot).toLowerCase();
+        if (/^\.(jpe?g|png|webp|gif|avif)$/.test(ext)) return ext;
+    }
+    const fromMime: Record<string, string> = {
+        'image/jpeg': '.jpg',
+        'image/png': '.png',
+        'image/webp': '.webp',
+        'image/gif': '.gif',
+        'image/avif': '.avif'
+    };
+    return fromMime[file.type] ?? '';
+}