feat(cards): 卡片外观可配置（不透明度 / 毛玻璃）

- 站点设置新增 card.opacity（40–100）与 card.blur，后台「首页管理 → 卡片外观」可调
- 前台所有卡片（首页 / 分组页 / 详情 / 侧栏）共用一套 CSS 变量
- 顺带带上上一轮：背景缓存、搜索记忆、明暗图标跟随实际状态、卡片描边/阴影、分组页卡片外壳

src/db/settings.rs

@@ -13,6 +13,8 @@ pub const SETTINGS_KEY: &str = "site";
 pub const MAX_HEADER_ITEMS: usize = 8;
 pub const MIN_SPEED: u32 = 8;
 pub const MAX_SPEED: u32 = 120;
+pub const MIN_CARD_OPACITY: u32 = 40;
+pub const MAX_CARD_OPACITY: u32 = 100;
 
 #[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq, Default)]
 #[serde(rename_all = "snake_case")]
@@ -119,6 +121,25 @@ pub struct BackgroundSettings {
     pub dark: String,
 }
 
+/// 卡片外观：透明度与毛玻璃。前台所有卡片共用一套（首页 / 分组页 / 详情页 / 侧栏）。
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(default)]
+pub struct CardSettings {
+    /// 卡片底色的不透明度（百分比，40–100）
+    pub opacity: u32,
+    /// 毛玻璃：背景模糊 + 提饱和度
+    pub blur: bool,
+}
+
+impl Default for CardSettings {
+    fn default() -> Self {
+        Self {
+            opacity: 100,
+            blur: false,
+        }
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, Default)]
 #[serde(default)]
 pub struct SearchEngineSettings {
@@ -288,6 +309,7 @@ pub struct Settings {
     pub header: HeaderSettings,
     pub sidebar: SidebarSettings,
     pub background: BackgroundSettings,
+    pub card: CardSettings,
     pub search: SearchSettings,
     pub profile: ProfileSettings,
     pub site: SiteMetaSettings,
@@ -354,6 +376,11 @@ pub fn validate(settings: &Settings) -> Result<(), StashError> {
             "暗色背景地址最多 500 个字符".into(),
         ));
     }
+    if !(MIN_CARD_OPACITY..=MAX_CARD_OPACITY).contains(&settings.card.opacity) {
+        return Err(StashError::InvalidDirectory(format!(
+            "卡片不透明度需在 {MIN_CARD_OPACITY}–{MAX_CARD_OPACITY} 之间"
+        )));
+    }
     if settings.search.engines.len() > 10 {
         return Err(StashError::InvalidDirectory("搜索引擎最多 10 个".into()));
     }
@@ -551,6 +578,14 @@ mod tests {
         let mut empty = Settings::default();
         empty.header.items.clear();
         assert!(save(&mut conn, empty, 0).is_err());
+
+        let mut too_transparent = Settings::default();
+        too_transparent.card.opacity = MIN_CARD_OPACITY - 1;
+        assert!(save(&mut conn, too_transparent, 0).is_err());
+
+        let mut clear = Settings::default();
+        clear.card.opacity = MAX_CARD_OPACITY;
+        assert!(save(&mut conn, clear, 0).is_ok());
     }
 
     #[test]