fix: prevent crash when clipboard is locked by another application

[Jefsky]

Problem

When another application holds the clipboard lock, calling Clipboard.SetText, Clipboard.GetText, or Clipboard.SetDataObject throws a COMException (error code 0x800401D0 - Clipboard can not be opened) and crashes the application.

Fix

Added safe wrapper methods around all clipboard operations with retry logic:

• SetClipboardTextSafe — wraps Clipboard.SetText in ServerSharingViewModel and OnlineConfigViewModel
• GetClipboardTextSafe — wraps Clipboard.GetText in MenuViewController for URL import
• SetClipboardDataObjectSafe — wraps Clipboard.SetDataObject in ShadowsocksController for PAC URL copy

Each wrapper retries up to 5 times with 50ms delay, gracefully failing silently instead of crashing.

Files Changed

• shadowsocks-csharp/ViewModels/ServerSharingViewModel.cs — QR code link copy
• shadowsocks-csharp/ViewModels/OnlineConfigViewModel.cs — online config link copy
• shadowsocks-csharp/View/MenuViewController.cs — URL import from clipboard
• shadowsocks-csharp/Controller/ShadowsocksController.cs — PAC URL copy

Test Plan

1. Open another app that holds the clipboard (e.g., a terminal with copied text)
2. Try all 4 clipboard operations in Shadowsocks:
   • Share Server Config → Copy Link
   • Online Config → Copy Link
   • Servers → Import URL from Clipboard
   • PAC → Copy Local PAC URL
3. App should not crash

[Jefsky]

Friendly bump — the PR has been sitting for a bit. The fix wraps clipboard access in try/catch across 4 sites (ShadowsocksController, MenuViewController, OnlineConfigViewModel, ServerSharingViewModel) to prevent the crash when the clipboard is locked by another app (~116 lines, 4 files). CI is green. Happy to address any feedback or split the changes per file if reviewers prefer.