revert: restore full config overwrite + Common Config Snippet (revert 992dda5c)

Revert the partial key-field merging refactoring introduced in 992dda5c,
along with two dependent commits (24fa8a18, 87604b18) that referenced
the now-removed ClaudeQuickToggles component.

The whitelist-based partial merge approach had critical issues:
- Non-whitelisted custom fields were lost during provider switching
- Backfill permanently stripped non-key fields from the database
- Whitelist required constant maintenance to track upstream changes

This restores the proven "full config overwrite + Common Config Snippet"
architecture where each provider stores its complete configuration and
shared settings are managed via a separate snippet mechanism.

Reverted commits:
- 24fa8a18: context-aware JSON editor hint + hide quick toggles
- 87604b18: hide ClaudeQuickToggles when creating
- 992dda5c: partial key-field merging refactoring

Restored:
- Full config snapshot write (write_live_snapshot) for Claude/Codex/Gemini
- Full config backfill (settings_config = live_config)
- Common Config Snippet UI and backend commands
- 6 frontend components/hooks for common config editing
- configApi barrel export and DB snippet methods

Removed:
- ClaudeQuickToggles component
- write_live_partial / backfill_key_fields / patch_claude_live
- All KEY_FIELDS constants

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

@@ -38,6 +38,31 @@ impl Database {
         Ok(())
     }
 
+    // --- 通用配置片段 (Common Config Snippet) ---
+
+    /// 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