feat(admin): 分组能力按类型收口（应用可分组，海报/图文平铺）

- 只有应用类分区显示分组管理与卡片预览分组；海报/图文保持平铺
- 后端 create_group 拒绝非应用分区；海报/图文读取时把历史分组条目一并平铺
- 侧栏不再显示非应用分组与空分组
- 类型 应用 ↔ 海报 切换时分组入口和侧栏子项随之出现/消失
- 测试 59 passed

src/db/categories.rs

@@ -361,17 +361,22 @@ pub fn create_group(conn: &mut Connection, section_id: &str, new: NewGroup) -> R
         return Err(StashError::InvalidDirectory("分组标题需为 1–40 个字".into()));
     }
 
-    let section_exists: Option<String> = conn
+    let view_mode: Option<String> = conn
         .query_row(
-            "SELECT key FROM node WHERE key = ?1 AND kind = 'section'",
+            "SELECT view_mode FROM node WHERE key = ?1 AND kind = 'section'",
             [section_id],
             |row| row.get(0),
         )
         .optional()
         .map_err(map_error)?;
 
-    if section_exists.is_none() {
+    let Some(view_mode) = view_mode else {
         return Err(StashError::NotFound(format!("分区不存在: {section_id}")));
+    };
+    if view_mode != "apps" {
+        return Err(StashError::InvalidDirectory(
+            "只有应用类分区可以添加分组".into(),
+        ));
     }
 
     let exists: i64 = conn
@@ -707,18 +712,21 @@ mod tests {
     }
 
     #[test]
-    fn groups_under_any_kind_of_section() {
+    fn groups_only_under_apps_sections() {
         let mut conn = db();
         create(&mut conn, section("films", "电影", "posters")).expect("应能创建");
-        create_group(
-            &mut conn,
-            "films",
-            NewGroup {
-                id: "g1".into(),
-                title: "片单".into(),
-            },
-        )
-        .expect("海报分区也应能建分组");
+        assert!(
+            create_group(
+                &mut conn,
+                "films",
+                NewGroup {
+                    id: "g1".into(),
+                    title: "片单".into()
+                }
+            )
+            .is_err(),
+            "海报 / 图文分区不提供分组"
+        );
     }
 
     #[test]