refactor(web): validate and repair diagram XML with the MCP server's engine

- Delete the web app's own copy of the XML checks and repairs from
  lib/utils.ts (1,074 lines). loadDiagram now uses the MCP server's
  validateAndFixXml without the strict checks, because the XML may hold
  the user's own diagram
- display_diagram and append_diagram prepare the model's XML with the new
  shared prepareNewDiagram, also used by the MCP create_new_diagram: wrap,
  validate strictly and auto-fix while it is still a bare model (where
  duplicate ids are renamed), then turn it into an mxfile
- The streaming preview of display_diagram no longer redraws the model's
  raw cells after the tool handler loaded the checked diagram, and drops
  a queued preview once the input is complete. That redraw lost
  auto-fixes and UserObject/object wrappers, so a linked cell lost its
  label; it also showed a second error toast
- The web repair regression tests now run against the MCP functions
- New e2e test checks the canvas content after display_diagram
- Fix the e2e upload tests, whose file input locator also matched the
  template import input

lib/i18n/dictionaries/en.json

@@ -178,9 +178,6 @@
         "networkError": "Network error. Please check your connection.",
         "retryLimit": "Auto-retry limit reached ({max}). Please try again manually.",
         "continuationRetryLimit": "Continuation retry limit reached ({max}). The diagram may be too complex.",
-        "validationFailed": "Diagram validation failed. Please try regenerating.",
-        "malformedXml": "AI generated invalid diagram XML. Please try regenerating.",
-        "failedToProcess": "Failed to process diagram. Please try regenerating.",
         "sessionCorrupted": "Session data was corrupted. Starting fresh.",
         "failedToSave": "Failed to save messages to localStorage",
         "failedToRestore": "Failed to restore from localStorage",

lib/i18n/dictionaries/ja.json

@@ -178,9 +178,6 @@
         "networkError": "ネットワークエラー。接続を確認してください。",
         "retryLimit": "自動再試行制限に達しました（{max}）。手動で再試行してください。",
         "continuationRetryLimit": "継続再試行制限に達しました（{max}）。ダイアグラムが複雑すぎる可能性があります。",
-        "validationFailed": "ダイアグラムの検証に失敗しました。再生成してみてください。",
-        "malformedXml": "AI が無効なダイアグラム XML を生成しました。再生成してみてください。",
-        "failedToProcess": "ダイアグラムの処理に失敗しました。再生成してみてください。",
         "sessionCorrupted": "セッションデータが破損しました。最初からやり直します。",
         "failedToSave": "localStorage へのメッセージの保存に失敗しました",
         "failedToRestore": "localStorage からの復元に失敗しました",

lib/i18n/dictionaries/zh-Hant.json

@@ -178,9 +178,6 @@
         "networkError": "網路錯誤。請檢查您的連線。",
         "retryLimit": "已達自動重試限制（{max}）。請手動重試。",
         "continuationRetryLimit": "已達繼續重試限制（{max}）。圖表可能過於複雜。",
-        "validationFailed": "圖表驗證失敗。請嘗試重新產生。",
-        "malformedXml": "AI 產生的圖表 XML 無效。請嘗試重新產生。",
-        "failedToProcess": "無法處理圖表。請嘗試重新產生。",
         "sessionCorrupted": "工作階段資料已損壞。重新開始。",
         "failedToSave": "無法儲存訊息到 localStorage",
         "failedToRestore": "無法從 localStorage 還原",

lib/i18n/dictionaries/zh.json

@@ -178,9 +178,6 @@
         "networkError": "网络错误。请检查您的连接。",
         "retryLimit": "已达到自动重试限制（{max}）。请手动重试。",
         "continuationRetryLimit": "已达到继续重试限制（{max}）。图表可能过于复杂。",
-        "validationFailed": "图表验证失败。请尝试重新生成。",
-        "malformedXml": "AI 生成的图表 XML 无效。请尝试重新生成。",
-        "failedToProcess": "无法处理图表。请尝试重新生成。",
         "sessionCorrupted": "会话数据已损坏。重新开始。",
         "failedToSave": "无法保存消息到 localStorage",
         "failedToRestore": "无法从 localStorage 恢复",