Add Hardcover shelf sync & unify book mappings

Introduce Hardcover provider support and consolidate per-provider book mapping tables into a unified BookMapping model. Adds two Prisma migrations (add_hardcover_shelves, unify_book_mappings), new backend services (hardcover-api, shelf-sync-core), and provider-specific sync logic and API routes for hardcover shelves with token/list validation. Frontend: new HardcoverForm component, refactor AddShelfModal to support Hardcover, hook updates, and small UI/accessibility tweaks. Also add documentation for Goodreads and Hardcover sync flows and update tests to cover scheduler/prisma helpers.

src/lib/utils/shelf-helpers.ts

@@ -0,0 +1,36 @@
+/**
+ * Component: Shelf Helpers
+ * Documentation: documentation/frontend/components.md
+ */
+
+/**
+ * Parse a JSON string of cover/book data into a typed array.
+ * Returns an empty array on parse failure (graceful degradation).
+ */
+export function processBooks(
+  coverUrls: string | null,
+): { coverUrl: string; asin: string | null; title: string; author: string }[] {
+  if (!coverUrls) return [];
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(coverUrls);
+  } catch {
+    return [];
+  }
+
+  if (!Array.isArray(parsed)) return [];
+
+  return parsed.map((item: unknown) => {
+    if (typeof item === 'string') {
+      return { coverUrl: item, asin: null, title: '', author: '' };
+    }
+    const obj = item as Record<string, unknown>;
+    return {
+      coverUrl: (obj.coverUrl as string) || '',
+      asin: (obj.asin as string) || null,
+      title: (obj.title as string) || '',
+      author: (obj.author as string) || '',
+    };
+  });
+}