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.test.ts

@@ -7,7 +7,12 @@ import {
     afterEach,
     type MockInstance
 } from 'vitest';
-import { listChapters, getChapter, getChapterPages } from './chapters';
+import {
+    listChapters,
+    getChapter,
+    getChapterPages,
+    createChapter
+} from './chapters';
 
 function ok(body: unknown): Response {
     return new Response(JSON.stringify(body), {
@@ -87,6 +92,43 @@ describe('chapters api client', () => {
         });
     });
 
+    it('createChapter POSTs multipart and renames page files to page-NNN.<ext>', async () => {
+        fetchSpy.mockResolvedValueOnce(ok({ ...chapterFixture, page_count: 3 }));
+        const pages = [
+            new File([new Uint8Array([1, 2])], 'IMG_2837.HEIC', { type: 'image/jpeg' }),
+            new File([new Uint8Array([3, 4])], 'random.png', { type: 'image/png' }),
+            // No extension; MIME-derived fallback should kick in.
+            new File([new Uint8Array([5])], 'scan_42', { type: 'image/webp' })
+        ];
+        const result = await createChapter(
+            'm1',
+            { number: 1, title: null },
+            pages
+        );
+        expect(result.page_count).toBe(3);
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/mangas\/m1\/chapters$/);
+        const init = fetchSpy.mock.calls[0][1] as RequestInit;
+        expect(init.method).toBe('POST');
+        const form = init.body as FormData;
+        // Metadata part is JSON.
+        const metadata = form.get('metadata') as Blob;
+        expect(metadata.type).toBe('application/json');
+        // Three pages, all renamed; original filenames discarded.
+        const submitted = form.getAll('page') as File[];
+        expect(submitted).toHaveLength(3);
+        // Original-extension preferred over MIME-derived; capitalised
+        // .HEIC dropped because it's not in the allowed list, so the
+        // MIME-derived `.jpg` wins.
+        expect(submitted[0].name).toBe('page-001.jpg');
+        expect(submitted[1].name).toBe('page-002.png');
+        expect(submitted[2].name).toBe('page-003.webp');
+        // No original filenames leak through.
+        for (const f of submitted) {
+            expect(f.name).not.toMatch(/IMG_2837|random|scan_42/);
+        }
+    });
+
     it('getChapterPages unwraps the {pages} envelope into the array', async () => {
         fetchSpy.mockResolvedValueOnce(
             ok({