Stop silent save failures from locking out textbook re-seeds

Two bugs were causing chapters to disappear on every relaunch:

1. seedTextbookData used `try? context.save()` (swallowing errors) and
   returned `inserted > 0`, so a failed save still reported success.
   Callers then bumped UserDefaults textbookDataVersion and subsequent
   launches skipped the re-seed entirely — with no rows on disk.

2. refreshTextbookDataIfNeeded wiped chapters via the batch-delete API
   `context.delete(model: TextbookChapter.self)`, which hits the store
   directly without clearing the context's .unique-id index. Re-inserting
   chapters with the same ids could then throw a unique-constraint error
   on save — also silently eaten by `try?`.

Fixes:
- seedTextbookData now uses do/catch around save(), returns false on
  error, and verifies persistence via fetchCount before returning true.
- refreshTextbookDataIfNeeded fetches and deletes chapters individually
  so the context tracks the deletion cleanly; wipe save is also now
  checked and bails early on failure.
- Bumped textbookDataVersion to 11 so devices poisoned by the previous
  silent-failure path retry on next launch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Conjuga/Conjuga/Services/DataLoader.swift

@@ -6,7 +6,7 @@ actor DataLoader {
     static let courseDataVersion = 7
     static let courseDataKey = "courseDataVersion"
 
-    static let textbookDataVersion = 10
+    static let textbookDataVersion = 11
     static let textbookDataKey = "textbookDataVersion"
 
     /// Quick check: does the DB need seeding or course data refresh?
@@ -156,17 +156,26 @@ actor DataLoader {
         print("Textbook data version outdated — re-seeding...")
         let context = ModelContext(container)
 
-        // Only wipe textbook chapters and our textbook-scoped CourseDecks
-        // (not the LanGo decks, which live in the same tables).
-        try? context.delete(model: TextbookChapter.self)
+        // Fetch + delete individually instead of batch delete. SwiftData's
+        // context.delete(model:) hits the store directly and doesn't always
+        // clear the unique-constraint index before the reseed's save runs,
+        // so re-inserting rows with the same .unique id can throw.
         let textbookCourseName = "Complete Spanish Step-by-Step"
+        if let existing = try? context.fetch(FetchDescriptor<TextbookChapter>()) {
+            for chapter in existing { context.delete(chapter) }
+        }
         let deckDescriptor = FetchDescriptor<CourseDeck>(
             predicate: #Predicate<CourseDeck> { $0.courseName == textbookCourseName }
         )
         if let decks = try? context.fetch(deckDescriptor) {
             for deck in decks { context.delete(deck) }
         }
-        try? context.save()
+        do {
+            try context.save()
+        } catch {
+            print("[DataLoader] ERROR: textbook wipe save failed: \(error)")
+            return
+        }
 
         if seedTextbookData(context: context) {
             shared.set(textbookDataVersion, forKey: textbookDataKey)
@@ -480,14 +489,27 @@ actor DataLoader {
             inserted += 1
         }
 
-        try? context.save()
+        do {
+            try context.save()
+        } catch {
+            print("[DataLoader] ERROR: textbook chapter save failed: \(error)")
+            return false
+        }
+
+        // Verify rows actually hit the store — guards against the case where
+        // save returned cleanly but no rows were persisted.
+        let persisted = (try? context.fetchCount(FetchDescriptor<TextbookChapter>())) ?? 0
+        guard persisted > 0 else {
+            print("[DataLoader] ERROR: textbook seeded \(inserted) chapters but persisted count is 0")
+            return false
+        }
 
         // Seed textbook-derived vocabulary flashcards as CourseDecks so the
         // existing Course UI can surface them alongside LanGo decks.
         seedTextbookVocabDecks(context: context, courseName: courseName)
 
-        print("Textbook seeding complete: \(inserted) chapters")
-        return inserted > 0
+        print("Textbook seeding complete: \(inserted) chapters inserted, \(persisted) persisted")
+        return true
     }
 
     private static func seedTextbookVocabDecks(context: ModelContext, courseName: String) {