revert(cards): 撤掉卡片外观配置与分组页卡片外壳，改用文字对比度

- 回退 card.opacity / card.blur 设置与后台「卡片外观」面板
- 分组页内容恢复为直接压在背景图上，不再套卡片
- 分组页文字：次级文字提一档 + 同主题描边（深色暗描边、浅色亮描边）
- 保留上一轮：背景缓存、搜索记忆、明暗图标、卡片描边/阴影、侧栏实底

src/db/settings.rs

@@ -13,8 +13,6 @@ 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")]
@@ -121,25 +119,6 @@ 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 {
@@ -309,7 +288,6 @@ pub struct Settings {
     pub header: HeaderSettings,
     pub sidebar: SidebarSettings,
     pub background: BackgroundSettings,
-    pub card: CardSettings,
     pub search: SearchSettings,
     pub profile: ProfileSettings,
     pub site: SiteMetaSettings,
@@ -376,11 +354,6 @@ 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()));
     }
@@ -578,14 +551,6 @@ 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]