feat: notifications modal, admin dedup, and in-app notification preferences

- Replace bell dropdown with full-screen NotificationsModal (mirrors SearchModal pattern)
- Notifications visible to all logged-in users (not just admin)
- Admin users excluded from new-chapter fan-out (dedup vs Scrape Complete notification)
- Users with notify_new_chapters=false opted out of new-chapter in-app notifications
- Toggle in profile page to enable/disable in-app new-chapter notifications
- PATCH /api/profile endpoint to save notification preferences
- User-facing /notifications page (admin redirects to /admin/notifications)

backend/internal/runner/runner.go

@@ -539,7 +539,21 @@ func (r *Runner) runScrapeTask(ctx context.Context, task domain.ScrapeTask) {
 				fmt.Sprintf("Scraped %d chapters, skipped %d (%s)", result.ChaptersScraped, result.ChaptersSkipped, task.Kind),
 				"/admin/tasks")
 		}
-		// Send push notifications to users who have this book in their library.
+		// Fan-out in-app new-chapter notification to all users who have this book
+		// in their library. Runs in background so it doesn't block the task loop.
+		if r.deps.Store != nil && result.ChaptersScraped > 0 &&
+			result.Slug != "" && task.Kind != "catalogue" {
+			go func() {
+				notifyCtx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+				defer cancel()
+				title := result.Slug
+				_ = r.deps.Store.NotifyUsersWithBook(notifyCtx, result.Slug,
+					"New chapters available",
+					fmt.Sprintf("%d new chapter(s) added to %s", result.ChaptersScraped, title),
+					"/books/"+result.Slug)
+			}()
+		}
+		// Send Web Push notifications to subscribed browsers.
 		if r.deps.WebPush != nil && r.deps.Store != nil &&
 			result.ChaptersScraped > 0 && result.Slug != "" && task.Kind != "catalogue" {
 			go r.deps.WebPush.SendToBook(context.Background(), r.deps.Store, result.Slug, webpush.Payload{