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/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 Pencil from '@lucide/svelte/icons/pencil';
     import UploadCloud from '@lucide/svelte/icons/upload-cloud';
 
     let { data } = $props();
@@ -327,6 +328,14 @@
                         <FolderPlus size={16} aria-hidden="true" />
                         <span>Add to collection</span>
                     </button>
+                    <a
+                        class="action"
+                        href="/manga/{manga.id}/edit"
+                        data-testid="edit-manga-link"
+                    >
+                        <Pencil size={16} aria-hidden="true" />
+                        <span>Edit</span>
+                    </a>
                     <a
                         class="action"
                         href="/manga/{manga.id}/upload-chapter"