refactor: remove OMO common config two-layer merge system

Each OMO provider now stores its complete configuration directly in
settings_config.otherFields instead of relying on a shared OmoGlobalConfig
merged at write time. This simplifies the data flow from a 4-tuple
(agents, categories, otherFields, useCommonConfig) to a 3-tuple and
eliminates an entire DB table, two Tauri commands, and ~1700 lines of
merge/sync code across frontend and backend.

Backend:
- Delete database/dao/omo.rs (OmoGlobalConfig struct + get/save methods)
- Remove get/set_config_snippet from settings DAO
- Remove get/set_common_config_snippet Tauri commands
- Replace merge_config() with build_config() in services/omo.rs
- Simplify OmoVariant (remove config_key, known_keys)
- Simplify import_from_local and build_local_file_data
- Rewrite all OMO service tests

Frontend:
- Delete OmoCommonConfigEditor.tsx and OmoGlobalConfigFields.tsx
- Delete src/lib/api/config.ts
- Remove OmoGlobalConfig type and merge preview functions
- Remove useGlobalConfig/useSaveGlobalConfig query hooks
- Simplify useOmoDraftState (remove all common config state)
- Replace OmoCommonConfigEditor with read-only JsonEditor preview
- Clean i18n keys (zh/en/ja)

src-tauri/src/database/dao/settings.rs

@@ -38,31 +38,6 @@ impl Database {
         Ok(())
     }
 
-    // --- Config Snippets 辅助方法 ---
-
-    /// 获取通用配置片段
-    pub fn get_config_snippet(&self, app_type: &str) -> Result<Option<String>, AppError> {
-        self.get_setting(&format!("common_config_{app_type}"))
-    }
-
-    /// 设置通用配置片段
-    pub fn set_config_snippet(
-        &self,
-        app_type: &str,
-        snippet: Option<String>,
-    ) -> Result<(), AppError> {
-        let key = format!("common_config_{app_type}");
-        if let Some(value) = snippet {
-            self.set_setting(&key, &value)
-        } else {
-            // 如果为 None 则删除
-            let conn = lock_conn!(self.conn);
-            conn.execute("DELETE FROM settings WHERE key = ?1", params![key])
-                .map_err(|e| AppError::Database(e.to_string()))?;
-            Ok(())
-        }
-    }
-
     // --- 全局出站代理 ---
 
     /// 全局代理 URL 的存储键名