feat: cache manager and webapp manifest repository

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

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+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
+    }
+}

app/src/main/java/top/yeij/hearth/webapp/WebAppRepository.kt

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+package top.yeij.hearth.webapp
+
+import com.google.gson.Gson
+import top.yeij.hearth.cache.CacheManager
+
+// Web 应用清单条目
+// Web app manifest entry
+data class WebApp(val id: String, val name: String, val icon: String, val url: String)
+
+// 网络抽象接口：Task 10 的 CardRepository 也会复用
+// HTTP abstraction reused by Task 10 CardRepository
+interface HttpClient {
+    fun get(url: String): String?
+}
+
+// 清单拉取仓库：拉取成功缓存 body 并解析；失败回退缓存；无缓存返回空列表
+// Manifest repository: on success cache + parse; on failure fall back to cache; no cache -> empty list
+class WebAppRepository(
+    private val http: HttpClient,
+    private val cache: CacheManager,
+    private val manifestUrl: String,
+) {
+    private val gson = Gson()
+
+    fun fetchManifest(): List<WebApp> {
+        val body = http.get(manifestUrl)
+        if (body != null) {
+            cache.save(KEY, body)
+            return parse(body)
+        }
+        val cached = cache.load(KEY) ?: return emptyList()
+        return parse(cached)
+    }
+
+    private fun parse(body: String): List<WebApp> {
+        val root = gson.fromJson(body, Map::class.java)
+        val apps = root["apps"] as? List<*> ?: return emptyList()
+        return apps.mapNotNull { m ->
+            val map = m as? Map<*, *> ?: return@mapNotNull null
+            val id = map["id"] as? String ?: return@mapNotNull null
+            val name = map["name"] as? String ?: return@mapNotNull null
+            val icon = map["icon"] as? String ?: return@mapNotNull null
+            val url = map["url"] as? String ?: return@mapNotNull null
+            WebApp(id, name, icon, url)
+        }
+    }
+
+    companion object {
+        private const val KEY = "webapp-manifest"
+    }
+}

app/src/test/java/top/yeij/hearth/cache/CacheManagerTest.kt

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+package top.yeij.hearth.cache
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+import java.io.File
+
+class CacheManagerTest {
+    @Test
+    fun saveThenLoad_roundTrips() {
+        val dir = File(System.getProperty("java.io.tmpdir"), "hearth-cache-test")
+        val cm = CacheManager(dir)
+        cm.save("manifest", "{\"version\":1}")
+        assertEquals("{\"version\":1}", cm.load("manifest"))
+        dir.deleteRecursively()
+    }
+
+    @Test
+    fun load_missingKey_returnsNull() {
+        val dir = File(System.getProperty("java.io.tmpdir"), "hearth-cache-miss")
+        val cm = CacheManager(dir)
+        assertNull(cm.load("nonexistent"))
+        dir.deleteRecursively()
+    }
+}

app/src/test/java/top/yeij/hearth/webapp/WebAppRepositoryTest.kt

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+package top.yeij.hearth.webapp
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import top.yeij.hearth.cache.CacheManager
+import java.io.File
+
+class WebAppRepositoryTest {
+    @Test
+    fun fetchManifest_success_parsesApps() {
+        val http = object : HttpClient {
+            override fun get(url: String) = """{"version":1,"apps":[{"id":"a","name":"云音乐","icon":"x.svg","url":"http://x/a"}]}"""
+        }
+        val repo = WebAppRepository(http, CacheManager(File(System.getProperty("java.io.tmpdir"), "w1")), "http://fake")
+        val apps = repo.fetchManifest()
+        assertEquals(1, apps.size)
+        assertEquals("云音乐", apps[0].name)
+    }
+
+    @Test
+    fun fetchManifest_failure_usesCache() {
+        val dir = File(System.getProperty("java.io.tmpdir"), "w2")
+        val cm = CacheManager(dir)
+        // 预写缓存（模拟之前拉取成功留下的缓存）
+        // Pre-seed the cache (simulate a previously successful fetch)
+        cm.save("webapp-manifest", """{"version":1,"apps":[{"id":"b","name":"缓存","icon":"x","url":"u"}]}""")
+        // 网络失败
+        // Network failure
+        val http = object : HttpClient { override fun get(url: String): String? = null }
+        val repo = WebAppRepository(http, cm, "http://fake")
+        assertEquals("缓存", repo.fetchManifest()[0].name)
+        dir.deleteRecursively()
+    }
+
+    @Test
+    fun fetchManifest_failure_withoutCache_returnsEmpty() {
+        val dir = File(System.getProperty("java.io.tmpdir"), "w3")
+        val http = object : HttpClient { override fun get(url: String): String? = null }
+        val repo = WebAppRepository(http, CacheManager(dir), "http://fake")
+        assertTrue(repo.fetchManifest().isEmpty())
+        dir.deleteRecursively()
+    }
+}