feat(frontend): add client-side cover generator via dynamic foliate-js import

New cover-generator.ts module that dynamically imports foliate-js/view.js
only when cover generation is requested, keeping it out of the main bundle.

Supports all media types:
- PDF (fixed_layout): renders page 1 to canvas via view.renderer
- EPUB/CBZ (reflowable): extracts book.cover blob from parsed metadata
- Falls back to canvas-to-JPEG conversion for non-JPEG sources

web/src/cover-generator.ts

@@ -0,0 +1,82 @@
+import { showToast } from "./toast";
+
+function getToken(): string {
+  return localStorage.getItem("token") || "";
+}
+
+export async function generateCoverBlob(
+  fileUrl: string,
+  formatGroup: string,
+): Promise<Blob | null> {
+  try {
+    const View = (await import("foliate-js/view.js")).default;
+    const view = new View();
+
+    const resp = await fetch(fileUrl, {
+      headers: { Authorization: `Bearer ${getToken()}` },
+    });
+    if (!resp.ok) {
+      showToast("Failed to fetch book file for cover generation", "error");
+      return null;
+    }
+
+    const blob = await resp.blob();
+    const file = new File([blob], "book", { type: blob.type });
+
+    const pdfOptions =
+      formatGroup === "fixed_layout"
+        ? {
+            pdf: {
+              cMapUrl: "/static/vendor/pdfjs/cmaps/",
+              standardFontDataUrl: "/static/vendor/pdfjs/standard_fonts/",
+            },
+          }
+        : {};
+
+    await view.open(file, pdfOptions);
+
+    if (!view.book?.sections?.length) {
+      showToast("Could not read book sections", "error");
+      return null;
+    }
+
+    if (formatGroup === "fixed_layout") {
+      const canvas = document.createElement("canvas");
+      await view.renderer?.renderPage(view.book.sections[0], canvas);
+      return new Promise((resolve) => {
+        canvas.toBlob(
+          (b) => resolve(b),
+          "image/jpeg",
+          0.85,
+        );
+      });
+    }
+
+    const coverHref = view.book.cover;
+    if (coverHref) {
+      const coverBlob = await coverHref.blob();
+      if (coverBlob.type.startsWith("image/")) {
+        return coverBlob;
+      }
+      return new Promise((resolve) => {
+        const img = new Image();
+        img.onload = () => {
+          const c = document.createElement("canvas");
+          c.width = img.naturalWidth;
+          c.height = img.naturalHeight;
+          c.getContext("2d")?.drawImage(img, 0, 0);
+          c.toBlob((b) => resolve(b), "image/jpeg", 0.85);
+        };
+        img.onerror = () => resolve(null);
+        img.src = URL.createObjectURL(coverBlob);
+      });
+    }
+
+    showToast("No cover found in book file", "error");
+    return null;
+  } catch (e) {
+    console.error("Cover generation failed:", e);
+    showToast("Cover generation failed", "error");
+    return null;
+  }
+}