Add workflow execution board design assets

[potter-sun]

Summary

Refs #2333.

Adds the Pencil design source and exported PNG previews for the low-interaction workflow execution display board.

Design Assets

• Pencil source: docs/designs/issue-2333-workflow-execution-board/aevatar-dashboard.pen
• Selection page preview: docs/designs/issue-2333-workflow-execution-board/aevatar-dashboard-exports-v17/mnkzC.png
• Display board preview: docs/designs/issue-2333-workflow-execution-board/aevatar-dashboard-exports-v17/V0WRu.png

Notes

• Both the setup/selection page and the display board use a dark, high-contrast Aevatar-aligned visual direction for large-screen readability.
• The setup page remains a simple close-range configuration step: select Team/Member scope, confirm, then enter the board.
• The display board is presentation-focused and avoids heavy interaction such as tabs, search, side filters, or frequent switching controls.
• The design preserves Team -> Member -> workflow execution node hierarchy and avoids implying memberId === workflowId.

Validation

• Pencil layout validation passed for both pages after the dark setup-page update.
• No code changes; build/test not run.