2D view: sun and moon for dark mode

[kbeck01]

Summary

• Adds a ☀️ / 🌙 button beside 🏢 3D on the 2D view. The sun means the page is light. The moon means it is dark.
• The browser keeps the choice. Cards, the bar, and the windows opened from the page use the night colors, with the same thick borders.

Test plan

• Open /lite. The sun sits immediately left of 🏢 3D.
• Click it. The page goes dark and the button becomes a moon. Click again and the page is light.
• Reload. The page stays in the mode you left it.
• Open a worker, the queue, and a board in dark mode and check the text is still readable.

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