refactor: 清理 LOW 问题 — 样式去重、死代码删除、命名修正

- 抽取 buttons.ts 共享按钮样式，消除 3 个组件的重复定义
- store 删除未调用的 canUndo/canRedo 方法
- importFromContent 变量 ext→lower 修正确性
- CSV 导出修复 BOM 重复（exportToCsv 自带 BOM）
- Rust error.rs 添加 allow(dead_code) 消除编译警告

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/store/app-store.ts

@@ -39,8 +39,6 @@ interface AppState {
 
   undo: () => void;
   redo: () => void;
-  canUndo: () => boolean;
-  canRedo: () => boolean;
 
   loadPaths: () => Promise<void>;
   savePaths: () => Promise<void>;
@@ -227,9 +225,6 @@ export const useAppStore = create<AppState>((set, get) => ({
     set({ isModified: !(arraysEqual(sysPaths, _savedSys) && arraysEqual(userPaths, _savedUser)) });
   },
 
-  canUndo: () => get().undoRedo.canUndo(),
-  canRedo: () => get().undoRedo.canRedo(),
-
   loadPaths: async () => {
     try {
       set({ isLoading: true });