[Fix] Make dashboard builder context menu keyboard accessible

[yuki-uix]

Summary

• render Expression Builder context-menu actions as semantic menu-item buttons
• focus the first action when the menu opens and support Arrow Up/Down, Home, End, and Escape
• restore focus after Escape and add visible keyboard focus styling
• add regression coverage for menu semantics and keyboard behavior

Closes #3515

Validation

• npm run test:unit (736 tests passed)
• npm run type-check
• npm run build
• targeted Vitest accessibility tests (10 tests passed)
• changed-file ESLint (0 errors; 3 pre-existing warnings in ExpressionBuilderInner.tsx)

Note: make dashboard-check completed the frontend lint stage, then stopped because golangci-lint is not installed in the local environment.

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