Develop (#55)

feat: optimize dashboard/search UX and simplify extension architecture

Redesign Dashboard with compact tool grid and recently used tools
Improve TopBar search UX with Cmd/Ctrl+K shortcut and better history navigation
Reorganize project structure into src/
Migrate i18n from react-i18next to chrome.i18n
Remove runtime language switch and settings page
Remove HTML/Markdown conversion tools
Clean up unused code, dead animations, redundant comments, and imports
Improve component consistency with shadcn/ui patterns
Replace hardcoded strings/colors with i18n tokens and theme tokens
Add comprehensive project documentation and coding standards
Fix CI artifact upload workflow and multiple TypeScript/test issues

Includes various refactors, UI polish, i18n cleanup, CI improvements,
and maintenance updates across the codebase.

src/lib/README.md

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+# lib/
+
+通用库工具目录，存放与业务无关的底层工具函数。
+
+## 文件说明
+
+| 文件       | 用途                                 |
+| ---------- | ------------------------------------ |
+| `utils.ts` | `cn()` 函数 — shadcn/ui 标准工具函数 |
+
+## cn()
+
+组合 `clsx` + `tailwind-merge`，用于合并和去重 Tailwind CSS 类名：
+
+```typescript
+import { cn } from '@/lib/utils';
+
+// 条件类名 + 合并外部 className
+<div className={cn(
+  'flex items-center gap-3 px-3',
+  isActive && 'bg-primary text-primary-foreground',
+  className,
+)}>
+```
+
+所有需要动态合并 Tailwind 类名的场景都应使用 `cn()`，而非手动拼接字符串。

src/lib/utils.ts

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+import { type ClassValue, clsx } from 'clsx';
+import { twMerge } from 'tailwind-merge';
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs));
+}