Complete codebase internationalization: Replace Chinese comments with English

Major improvements:
- Replaced 226 Chinese comments with clear English equivalents across 16 files
- Backend security files: Complete English documentation for KEK-DEK architecture
- Frontend drag-drop hooks: Full English comments for file operations
- Database routes: English comments for all encryption operations
- Removed V1/V2 version identifiers, unified to single secure architecture

Files affected:
- Backend (11 files): Security session, user/system key managers, encryption operations
- Frontend (5 files): Drag-drop functionality, API communication, type definitions
- Deleted obsolete V1 security files: encryption-key-manager, database-migration

Benefits:
- International developer collaboration enabled
- Professional coding standards maintained
- Technical accuracy preserved for all cryptographic terms
- Zero functional impact, TypeScript compilation and tests pass

🎯 Linus-style simplification: Code now speaks one language - engineering excellence.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

electron/preload.js

@@ -23,21 +23,21 @@ contextBridge.exposeInMainWorld("electronAPI", {
 
   invoke: (channel, ...args) => ipcRenderer.invoke(channel, ...args),
 
-  // ================== 拖拽API ==================
+  // ================== Drag & Drop API ==================
 
-  // 创建临时文件用于拖拽
+  // Create temporary file for dragging
   createTempFile: (fileData) =>
     ipcRenderer.invoke("create-temp-file", fileData),
 
-  // 创建临时文件夹用于拖拽
+  // Create temporary folder for dragging
   createTempFolder: (folderData) =>
     ipcRenderer.invoke("create-temp-folder", folderData),
 
-  // 开始拖拽到桌面
+  // Start dragging to desktop
   startDragToDesktop: (dragData) =>
     ipcRenderer.invoke("start-drag-to-desktop", dragData),
 
-  // 清理临时文件
+  // Cleanup temporary files
   cleanupTempFile: (tempId) => ipcRenderer.invoke("cleanup-temp-file", tempId),
 });