feat: bundle demo webapp in apk

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

@@ -44,15 +44,37 @@ class WebAppRepository(
             // 先解析、成功才缓存，避免畸形 body 或空结果被永久缓存
             // Parse first, cache only on success (never cache malformed/empty bodies)
             val apps = parse(body)
-            if (apps != null) {
+            if (apps != null && apps.isNotEmpty()) {
                 cache.save(KEY, body)
                 return apps
             }
-            Log.w(TAG, "fetchManifest: invalid body, not cached")
-            return emptyList()
+            Log.w(TAG, "fetchManifest: invalid/empty body, not cached")
+            return builtinWebApps()
         }
-        val cached = cache.load(KEY) ?: return emptyList()
-        return parse(cached) ?: emptyList()
+        val cached = cache.load(KEY)
+        if (cached != null) {
+            val apps = parse(cached)
+            if (apps != null && apps.isNotEmpty()) return apps
+        }
+        // 无远程清单：返回内置示例 webapp，保证列表不为空、可演示
+        // No remote manifest: return the builtin demo webapp so the list is non-empty
+        return builtinWebApps()
+    }
+
+    // 内置示例 webapp（打包在 assets 内，无需服务器）
+    // Builtin demo webapp (bundled in assets, no server required)
+    private fun builtinWebApps(): List<WebApp> {
+        return listOf(
+            WebApp(
+                id = "demo",
+                name = "示例应用",
+                icon = "data:image/svg+xml;utf8," +
+                    "<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24'>" +
+                    "<rect width='24' height='24' rx='6' fill='%23ff6900'/>" +
+                    "<text x='12' y='17' font-size='13' text-anchor='middle' fill='white'>H</text></svg>",
+                url = "file:///android_asset/webapps/demo/index.html",
+            )
+        )
     }
 
     // 解析清单 JSON；畸形 JSON 或缺少 apps 字段返回 null（调用方决定回退）