Add VLM-based diagram validation (#602)

* [Feature] Add VLM-based diagram validation

Add automatic VLM (Vision Language Model) validation after display_diagram
tool execution. The system captures a screenshot of the rendered diagram,
sends it to a VLM for visual analysis, and uses feedback to improve
diagram quality through the existing retry mechanism.

Changes:
- Add /api/validate-diagram endpoint for VLM validation
- Add diagram-validator.ts for client-side validation orchestration
- Add validation-prompts.ts for VLM system prompts
- Add ValidationCard component to display validation status in chat
- Add PNG capture functionality to diagram context
- Integrate validation into tool handlers with retry support (max 3)
- Add "Improve with Suggestions" button for manual regeneration
- Add settings toggle to enable/disable VLM validation
- Add getValidationModel() helper in ai-providers.ts

* refactor(validation): use AI SDK structured outputs and address review feedback

- Replace generateText + manual JSON parsing with generateObject and Zod schema
  for type-safe structured validation output
- Use AbortSignal.timeout() instead of Promise.race for cleaner timeout handling
- Add timeout validation with minimum 1000ms to handle malformed env values
- Remove unused xml parameter from validateRenderedDiagram API
- Remove parseValidationResponse function (now handled by schema)
- Clear validationStates on session switch and new chat to prevent memory leak
- Update 100ms render delay comment to clarify best-effort heuristic
- Remove unused useEffect import from ValidationCard
- Fix optional chaining lint warning in ValidationCard
- Add unit tests for formatValidationFeedback function

* refactor(validation): use AI SDK experimental_useObject hook instead of raw fetch

- Change API endpoint from generateObject to streamObject for useObject compatibility
- Create useValidateDiagram hook using AI SDK's experimental_useObject for reactive validation
- Update useDiagramToolHandlers to accept validation function as parameter
- Update chat-panel to use new useValidateDiagram hook
- Remove validateRenderedDiagram function from lib/diagram-validator.ts (now in hook)
- Export ValidationResultSchema from API route for client-side use

* fix(validation): extract schema to shared file for client/server compatibility

Move ValidationResultSchema to lib/validation-schema.ts to avoid importing
server-side modules (ai-providers) into client-side code. This fixes the
Turbopack build error caused by the hook importing from the API route.

* fix(validation): use 'Valid' instead of 'Complete' for validation success

Change ValidationCard success label from 'Complete' to 'Valid' to avoid
conflicting with ToolCallCard's 'Complete' badge in E2E tests. This fixes
the diagram-generation E2E test that expects a specific count of 'Complete'
badges.

* fix(validation): add aria-hidden to icons to prevent duplicate ID warning

* fix: improve VLM validation with bug fixes and i18n

- Fix race condition in pendingValidationRef (reject previous pending validation)
- Fix response format consistency (use streaming for all responses)
- Remove dead code (unused lastRequestRef and ValidationRequest interface)
- Consolidate duplicate types (re-export from validation-schema.ts)
- Add 'success_with_warnings' status for valid diagrams with warnings
- Fix tool card auto-collapse (only collapse once, respect user toggle)
- Set VLM validation default to disabled
- Add i18n support for diagram validation settings (en/zh/ja)
- Mark feature as experimental in settings UI

* fix: resolve TypeScript errors in electron-standalone

- Add forwardRef support to ChatInput component with ChatInputRef type
- Copy electron.d.ts to electron-standalone/electron folder
- Exclude electron-standalone from root tsconfig type checking

* fix: return empty string for valid result with no issues in formatValidationFeedback

* feat(i18n): add validation strings for ValidationCard component

- Add validation section to en.json, zh.json, ja.json dictionaries
- Update ValidationCard to use useDictionary hook
- Replace all hardcoded English strings with i18n keys

---------

Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

lib/i18n/dictionaries/ja.json

@@ -115,7 +115,11 @@
         "httpProxy": "HTTP プロキシ",
         "httpsProxy": "HTTPS プロキシ",
         "applyProxy": "適用",
-        "proxyApplied": "プロキシ設定が適用されました"
+        "proxyApplied": "プロキシ設定が適用されました",
+        "diagramValidation": "ダイアグラム検証（実験的）",
+        "diagramValidationDescription": "視覚言語モデルを使用して生成されたダイアグラムを検証します。GPT-5.2 や Sonnet-4.5 などの VLM が必要です。",
+        "enabled": "有効",
+        "disabled": "無効"
     },
     "save": {
         "title": "ダイアグラムを保存",
@@ -248,6 +252,24 @@
         "searchPlaceholder": "チャットを検索...",
         "noResults": "チャットが見つかりません"
     },
+    "validation": {
+        "title": "ダイアグラムを検証",
+        "capturing": "キャプチャ中",
+        "validating": "検証中",
+        "validatingWithAttempt": "検証中 ({attempt}/{max})",
+        "valid": "有効",
+        "validWithWarnings": "有効（警告あり）",
+        "issuesFound": "問題が見つかりました",
+        "error": "エラー",
+        "skipped": "スキップ",
+        "capturedScreenshot": "キャプチャした画像：",
+        "issuesFoundLabel": "検出された問題：",
+        "suggestions": "提案：",
+        "passedValidation": "ダイアグラムは視覚検証に合格しました - 問題は検出されませんでした。",
+        "improvementRequested": "改善リクエスト済み - 下の新しいダイアグラムを確認してください",
+        "improveWithSuggestions": "提案で改善",
+        "regenerateWithFeedback": "検証フィードバックを使用してダイアグラムを再生成"
+    },
     "modelConfig": {
         "title": "AIモデル設定",
         "description": "複数のAIプロバイダーとモデルを設定",