feat: edit existing manga metadata (0.31.0)

Adds PUT /mangas/:id/cover (multipart) and DELETE /mangas/:id/cover so
covers can be replaced or cleared after creation, and wires a dedicated
/manga/[id]/edit SvelteKit route that combines the existing PATCH with
the new cover endpoints. Cover PUT cleans up the old blob when the
extension changes, swallowing StorageError::NotFound so a manually-gone
file doesn't surface as a 404 to the client. Edit link on the manga
detail page is gated on session.user, matching the auth posture of the
underlying handlers.

Also pins the local-dev port story via loadEnv() in vite.config.ts so
VITE_PORT / BACKEND_URL from a (gitignored) .env keep the dev URL
stable across runs.

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

frontend/src/lib/api/mangas.test.ts

@@ -4,6 +4,8 @@ import {
     createManga,
     getManga,
     updateManga,
+    updateMangaCover,
+    deleteMangaCover,
     attachTag,
     detachTag
 } from './mangas';
@@ -184,6 +186,49 @@ describe('mangas api client', () => {
         });
     });
 
+    it('updateMangaCover PUTs multipart with the cover blob', async () => {
+        fetchSpy.mockResolvedValueOnce(
+            ok(detailFixture({ cover_image_path: 'mangas/b1/cover.png' }))
+        );
+        const cover = new Blob([new Uint8Array([0x89, 0x50, 0x4e, 0x47])], { type: 'image/png' });
+        const updated = await updateMangaCover('b1', cover);
+        expect(updated.cover_image_path).toBe('mangas/b1/cover.png');
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/mangas\/b1\/cover$/);
+        const init = fetchSpy.mock.calls[0][1] as RequestInit;
+        expect(init.method).toBe('PUT');
+        expect(init.body).toBeInstanceOf(FormData);
+        const form = init.body as FormData;
+        expect(form.get('cover')).toBeInstanceOf(Blob);
+        // Boundary is filled in by the browser when body is FormData.
+        expect(init.headers).toBeUndefined();
+    });
+
+    it('updateMangaCover throws ApiError on payload_too_large', async () => {
+        fetchSpy.mockResolvedValue(
+            envelope(413, 'payload_too_large', 'cover exceeds size cap')
+        );
+        const cover = new Blob([new Uint8Array(1)]);
+        await expect(updateMangaCover('b1', cover)).rejects.toMatchObject({
+            name: 'ApiError',
+            status: 413,
+            code: 'payload_too_large'
+        });
+    });
+
+    it('deleteMangaCover DELETEs and returns the refreshed detail with null path', async () => {
+        fetchSpy.mockResolvedValueOnce(
+            ok(detailFixture({ cover_image_path: null }))
+        );
+        const updated = await deleteMangaCover('b1');
+        expect(updated.cover_image_path).toBeNull();
+        const url = fetchSpy.mock.calls[0][0] as string;
+        expect(url).toMatch(/\/v1\/mangas\/b1\/cover$/);
+        const init = fetchSpy.mock.calls[0][1] as RequestInit;
+        expect(init.method).toBe('DELETE');
+        expect(init.body).toBeUndefined();
+    });
+
     it('attachTag POSTs the name and returns the TagRef', async () => {
         fetchSpy.mockResolvedValueOnce(
             ok({ id: 't9', name: 'Dark Fantasy', added_by: 'u1' }, 201)