refactor(mcp): complete v3.7.0 cleanup - remove legacy code and warnings

This commit finalizes the v3.7.0 unified MCP architecture migration by
removing all deprecated code paths and eliminating compiler warnings.

Frontend Changes (~950 lines removed):
- Remove deprecated components: McpPanel, McpListItem, McpToggle
- Remove deprecated hook: useMcpActions
- Remove unused API methods: importFrom*, syncEnabledTo*, syncAllServers
- Simplify McpFormModal by removing dual-mode logic (unified/legacy)
- Remove syncOtherSide checkbox and conflict detection
- Clean up unused imports and state variables
- Delete associated test files

Backend Changes (~400 lines cleaned):
- Remove unused Tauri commands: import_mcp_from_*, sync_enabled_mcp_to_*
- Delete unused Gemini MCP functions: get_mcp_status, upsert/delete_mcp_server
- Add #[allow(deprecated)] to compatibility layer commands
- Add #[allow(dead_code)] to legacy helper functions for future migration
- Simplify boolean expression in mcp.rs per Clippy suggestion

API Deprecation:
- Mark legacy APIs with @deprecated JSDoc (getConfig, upsertServerInConfig, etc.)
- Preserve backward compatibility for v3.x, planned removal in v4.0

Verification:
- ✅ Zero TypeScript errors (pnpm typecheck)
- ✅ Zero Clippy warnings (cargo clippy)
- ✅ All code formatted (prettier + cargo fmt)
- ✅ Builds successfully

Total cleanup: ~1,350 lines of code removed/marked
Breaking changes: None (all legacy APIs still functional)

src-tauri/src/commands/config.rs

@@ -141,7 +141,10 @@ pub async fn open_app_config_folder(handle: AppHandle) -> Result<bool, String> {
 pub async fn get_claude_common_config_snippet(
     state: tauri::State<'_, crate::store::AppState>,
 ) -> Result<Option<String>, String> {
-    let guard = state.config.read().map_err(|e| format!("读取配置锁失败: {e}"))?;
+    let guard = state
+        .config
+        .read()
+        .map_err(|e| format!("读取配置锁失败: {e}"))?;
     Ok(guard.claude_common_config_snippet.clone())
 }