[Upstream #2163][WebDAV] Follow permanent endpoint redirects during backup and restore

[SingLinkNetwork]

Source

• Upstream: [BUG] 无法拾取和保存文件 chen08209/FlClash#2163
• Reported behavior: WebDAV restore returns Moved Permanently.

Scope

This tracks only the WebDAV permanent-redirect portion. The Android local file-picker portion is tracked separately in #97.

Root cause

The bundled WebDAV client retries 302 redirects itself but does not recognize HTTP 301. A canonical WebDAV endpoint that returns 301 is therefore treated as failed.

Fix and verification

• Normalize HTTP 301 to the client's existing safe 302 retry path, preserving the WebDAV method and request body.
• Regression test uses a local WebDAV-like endpoint that returns 301 for OPTIONS / and succeeds at the redirected path.
• Local full Flutter tests and static analysis pass.
• This issue remains open until the stacked draft PR passes full GitHub Actions and is merged/released.

[SingLinkNetwork]

已完成修復並通過完整驗證。

• 修復：WebDAV 端點回傳 HTTP 301（Moved Permanently）時，會走既有的安全重試流程，保留原本的 WebDAV 方法與傳輸內容。
• 回歸測試：本機啟動測試伺服器，令 OPTIONS / 回傳 301 後轉到正確端點並成功。
• 本機：完整 flutter test、flutter analyze --no-fatal-infos 與差異檢查通過；分析僅有既有 10 條 withOpacity 提示。
• 雲端：GitHub Actions run 31360137939 完整 63/63 通過，包含 Android、macOS、Linux、Windows x64、Windows ARM64 與合併 MSIX 安裝包。

對應草稿 PR：#100。問題保留開啟，直到修復鏈完成合併並進入正式版本。