feat: cache manager and webapp manifest repository

app/src/main/java/top/yeij/hearth/cache/CacheManager.kt

@@ -0,0 +1,18 @@
+package top.yeij.hearth.cache
+
+import java.io.File
+
+// 文件缓存：以 key 为文件名，内容以 UTF-8 纯文本落盘
+// File cache: uses key as filename, content stored as UTF-8 plain text
+class CacheManager(private val baseDir: File) {
+    init { baseDir.mkdirs() }
+
+    fun save(key: String, content: String) {
+        File(baseDir, key).writeText(content)
+    }
+
+    fun load(key: String): String? {
+        val f = File(baseDir, key)
+        return if (f.exists()) f.readText() else null
+    }
+}