fix(ui): remove the broken Generate Cover button from the metadata editor

Generate Cover has never worked: it fetched the book file using a URL
scraped from the cover preview <img> tag (so it downloaded either the
existing cover JPEG or, when no cover existed, the detail page HTML),
then handed it to foliate-js, which rejects both. Its fixed-layout path
also called view.renderer.renderPage(), a method that does not exist in
the pinned foliate fork. Every click ended in the same generic 'Cover
generation failed' toast.

The working alternative already exists server-side: the scanner's
PDF/EPUB cover extraction plus the per-book Rescan button, now that the
rasterizer renders the CropBox. Users who want a specific image can
upload one.

Delete web/src/cover-generator.ts, the modal buttons, and the dead
generateCover()/coverGenerating/fileUrl plumbing in book-detail.ts.

web/src/book-detail.ts

@@ -1,6 +1,5 @@
 import { Alpine } from "./alpine";
 import { showToast } from "./toast";
-import { generateCoverBlob } from "./cover-generator";
 import { searchTagSuggestions, type TagSuggestion } from "./tag-dropdown";
 
 function getMediaId(): string {
@@ -91,7 +90,6 @@ export {
 
 interface MetadataEditorState {
   openSections: Record<string, boolean>;
-  coverGenerating: boolean;
   hasExistingCover: boolean;
   newCoverPreview: string;
   coverFile: Blob | null;
@@ -108,7 +106,6 @@ interface MetadataEditorState {
   showMetadataEditor(): void;
   hideMetadataEditor(): void;
   handleCoverUpload(event: Event): void;
-  generateCover(): Promise<void>;
   removeCover(): void;
   saveMetadata(): Promise<void>;
   rescanBook(): Promise<void>;
@@ -130,12 +127,6 @@ Alpine.data("bookDetail", () => {
     coverImg?.src &&
     !coverImg.src.includes("placeholder-book.svg");
 
-  const coverPreviewEl = document.querySelector(
-    ".aspect-\\[2\\/3\\] img",
-  ) as HTMLImageElement | null;
-  const coverSrc = coverPreviewEl?.src || "";
-  const fileUrl = coverSrc && !coverSrc.includes("placeholder") ? coverSrc : "";
-
   const initialTags: string[] = [];
   const tagBadges = document.querySelectorAll("#metadata-editor-modal [data-editor-tag]");
   tagBadges.forEach((el) => {
@@ -145,7 +136,6 @@ Alpine.data("bookDetail", () => {
 
   return {
     openSections: { basic: true } as Record<string, boolean>,
-    coverGenerating: false,
     hasExistingCover: !!hasCover,
     newCoverPreview: "",
     coverFile: null as Blob | null,
@@ -462,33 +452,6 @@ Alpine.data("bookDetail", () => {
       reader.readAsDataURL(file);
     },
 
-    async generateCover() {
-      this.coverGenerating = true;
-      try {
-        const formatGroup =
-          document
-            .querySelector('[data-format-group]')
-            ?.getAttribute("data-format-group") || "reflowable";
-
-        const blob = await generateCoverBlob(fileUrl, formatGroup);
-        if (!blob) return;
-
-        this.coverFile = blob;
-        this.coverAction = "upload";
-
-        const preview = document.getElementById(
-          "metadata-cover-preview",
-        ) as HTMLImageElement;
-        if (preview) {
-          preview.src = URL.createObjectURL(blob);
-        }
-        this.newCoverPreview = URL.createObjectURL(blob);
-        showToast("Cover generated successfully", "success");
-      } finally {
-        this.coverGenerating = false;
-      }
-    },
-
     removeCover() {
       this.coverAction = "remove";
       this.coverFile = null;