add save function of explanation

ios/OralTrainer/Services/SentenceExplanationStore.swift

@@ -0,0 +1,47 @@
+import Foundation
+
+final class SentenceExplanationStore {
+    static let shared = SentenceExplanationStore()
+
+    private let root: URL
+
+    init() {
+        let documents = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
+        root = documents.appendingPathComponent("SentenceExplanations", isDirectory: true)
+        try? FileManager.default.createDirectory(at: root, withIntermediateDirectories: true)
+    }
+
+    func load(mediaId: String, sentenceIndex: Int) -> String? {
+        let url = fileURL(mediaId: mediaId, sentenceIndex: sentenceIndex)
+        guard FileManager.default.fileExists(atPath: url.path) else { return nil }
+        let text = try? String(contentsOf: url, encoding: .utf8)
+        let trimmed = text?.trimmingCharacters(in: .whitespacesAndNewlines) ?? ""
+        return trimmed.isEmpty ? nil : trimmed
+    }
+
+    @discardableResult
+    func save(mediaId: String, sentenceIndex: Int, explanation: String) -> Bool {
+        let trimmed = explanation.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !trimmed.isEmpty else { return false }
+        let url = fileURL(mediaId: mediaId, sentenceIndex: sentenceIndex)
+        try? FileManager.default.createDirectory(at: url.deletingLastPathComponent(), withIntermediateDirectories: true)
+        do {
+            try trimmed.write(to: url, atomically: true, encoding: .utf8)
+            return true
+        } catch {
+            return false
+        }
+    }
+
+    private func fileURL(mediaId: String, sentenceIndex: Int) -> URL {
+        let directory = root.appendingPathComponent(Self.sanitize(mediaId), isDirectory: true)
+        return directory.appendingPathComponent("\(sentenceIndex).txt")
+    }
+
+    static func sanitize(_ mediaId: String) -> String {
+        let allowed = CharacterSet.alphanumerics.union(CharacterSet(charactersIn: "._-"))
+        let filtered = mediaId.lowercased().unicodeScalars.map { allowed.contains($0) ? Character($0) : "_" }
+        let value = String(filtered).trimmingCharacters(in: CharacterSet(charactersIn: "_"))
+        return value.isEmpty || value == "." || value == ".." ? "unknown" : value
+    }
+}