Add 6 new practice features, offline dictionary, and feature reference

New features:
- Offline Dictionary: reverse index of 175K verb forms + 200 common
  words, cached to disk, powers instant word lookups in Stories
- Vocab SRS Review: spaced repetition for course vocabulary cards
  with due count badge and Again/Hard/Good/Easy rating
- Cloze Practice: fill-in-the-blank using SentenceQuizEngine with
  distractor generation from vocabulary pool
- Grammar Exercises: interactive quizzes for 5 grammar topics
  (ser/estar, por/para, preterite/imperfect, subjunctive, personal a)
  with "Practice This" button on grammar note detail
- Listening Practice: listen-and-type + pronunciation check modes
  using Speech framework with word-by-word match scoring
- Conversational Practice: AI chat partner via Foundation Models
  with 10 scenario types, saved to cloud container

Other changes:
- Add Conversation model to SharedModels and cloud container
- Add Info.plist keys for speech recognition and microphone
- Skip speech auth on simulator to prevent crash
- Fix preparing data screen to only show during seed/migration
- Extract courseDataVersion to static property on DataLoader
- Add "How Features Work" reference page in Settings

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

Conjuga/SharedModels/Sources/SharedModels/Conversation.swift

@@ -0,0 +1,48 @@
+import SwiftData
+import Foundation
+
+@Model
+public final class Conversation {
+    public var id: String = ""
+    public var scenario: String = ""
+    public var level: String = ""
+    public var messages: String = "[]"
+    public var createdDate: Date = Date()
+
+    public init(scenario: String, level: String) {
+        self.id = UUID().uuidString
+        self.scenario = scenario
+        self.level = level
+        self.messages = "[]"
+        self.createdDate = Date()
+    }
+}
+
+public struct ChatMessage: Codable, Identifiable, Hashable {
+    public var id: String
+    public let role: String    // "assistant" or "user"
+    public let content: String
+    public let correction: String?
+
+    public init(role: String, content: String, correction: String? = nil) {
+        self.id = UUID().uuidString
+        self.role = role
+        self.content = content
+        self.correction = correction
+    }
+}
+
+extension Conversation {
+    public var decodedMessages: [ChatMessage] {
+        guard let data = messages.data(using: .utf8) else { return [] }
+        return (try? JSONDecoder().decode([ChatMessage].self, from: data)) ?? []
+    }
+
+    public func appendMessage(_ message: ChatMessage) {
+        var msgs = decodedMessages
+        msgs.append(message)
+        if let data = try? JSONEncoder().encode(msgs), let str = String(data: data, encoding: .utf8) {
+            messages = str
+        }
+    }
+}