Implements "另存为" (Save As) context menu for files and folders, supporting both web browser downloads and desktop native save dialogs.

Changes

• UI: Added "另存为..." menu items to file and folder context menus
• File Save: Exports individual files with current content, preserving extensions
• Folder Save: Exports complete directory structures as JSON with all files, subfolders, content, project types, and NuGet configs
• Desktop Integration: Uses native OS save dialogs via desktopBridge.requestFileDownload() with save-file-as/save-folder-as actions
• Web Fallback: Triggers browser downloads when desktop bridge unavailable

Implementation

Created shared utilities to eliminate duplication (~130 lines):

• sanitizeFileName(): Strips path separators, control chars, limits length
• requestDesktopSave(): Centralized desktop bridge requests
• downloadBlob(): Unified browser download logic
• createFolderStructureData(): Recursively builds folder structure from latest localStorage content

Refactored existing exportFolder() to use shared methods for consistency.

Security

Filename sanitization prevents:

• Directory traversal (/, \, :)
• Control characters (\x00-\x1f)
• Hidden files (leading/trailing dots)
• Filesystem limits (200 char max)

Usage

// Right-click file → "另存为..."
saveFileAs() → downloads file.cs with latest content

// Right-click folder → "另存为..."  
saveFolderAs() → downloads folder.json with complete structure

Desktop mode shows saved path in notifications. Folders can be re-imported via existing "导入" functionality.

• Fixes feat # 另存为文件或目录到磁盘 #85

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