Add AI-generated short stories with tappable words and comprehension quiz

Generate one-paragraph Spanish stories on-device using Foundation Models,
matched to user's level and enabled tenses. Every word is tappable —
pre-annotated words show instantly, others get a quick on-device AI
lookup with caching. English translation hidden by default behind a
toggle. Comprehension quiz with 3 multiple-choice questions. Stories
saved to cloud container for sync and persistence across resets.

Closes #9

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Conjuga/SharedModels/Sources/SharedModels/Story.swift

@@ -0,0 +1,67 @@
+import SwiftData
+import Foundation
+
+@Model
+public final class Story {
+    public var id: String = ""
+    public var title: String = ""
+    public var bodyES: String = ""
+    public var bodyEN: String = ""
+    public var level: String = ""
+    public var wordAnnotations: String = "[]"
+    public var quizQuestions: String = "[]"
+    public var createdDate: Date = Date()
+
+    public init(title: String, bodyES: String, bodyEN: String, level: String, wordAnnotations: String, quizQuestions: String) {
+        self.id = UUID().uuidString
+        self.title = title
+        self.bodyES = bodyES
+        self.bodyEN = bodyEN
+        self.level = level
+        self.wordAnnotations = wordAnnotations
+        self.quizQuestions = quizQuestions
+        self.createdDate = Date()
+    }
+}
+
+// MARK: - JSON Helpers
+
+public struct WordAnnotation: Codable, Identifiable, Hashable {
+    public var id: String { word }
+    public let word: String
+    public let baseForm: String
+    public let english: String
+    public let partOfSpeech: String
+
+    public init(word: String, baseForm: String, english: String, partOfSpeech: String) {
+        self.word = word
+        self.baseForm = baseForm
+        self.english = english
+        self.partOfSpeech = partOfSpeech
+    }
+}
+
+public struct QuizQuestion: Codable, Identifiable, Hashable {
+    public var id: String { question }
+    public let question: String
+    public let options: [String]
+    public let correctIndex: Int
+
+    public init(question: String, options: [String], correctIndex: Int) {
+        self.question = question
+        self.options = options
+        self.correctIndex = correctIndex
+    }
+}
+
+extension Story {
+    public var decodedAnnotations: [WordAnnotation] {
+        guard let data = wordAnnotations.data(using: .utf8) else { return [] }
+        return (try? JSONDecoder().decode([WordAnnotation].self, from: data)) ?? []
+    }
+
+    public var decodedQuestions: [QuizQuestion] {
+        guard let data = quizQuestions.data(using: .utf8) else { return [] }
+        return (try? JSONDecoder().decode([QuizQuestion].self, from: data)) ?? []
+    }
+}