feat: update header to white background with black text

[devin-ai-integration]

Summary

Updates the header navigation styling from a red/maroon background with white text to a white background with black text, based on the Figma design reference.

Changes:

• Changed header background color from red ($theme-day-secondary-color) to white
• Updated all header text colors from white to black
• Added filter: invert(1) to the settings gear icon to make it visible on the white background
• Swapped logo inner color to use the secondary color for better contrast

Review & Testing Checklist for Human

• Verify the header visually matches expectations (white background, black nav text, visible gear icon)
• Test the settings gear icon - the filter: invert(1) approach inverts the white SVG; confirm it looks correct
• Switch to night/dark theme and verify it still works correctly (these themes were not modified)

Recommended test plan: Run the app locally with npm start and visually inspect the header in the default day theme. Click through the navigation links and settings icon to ensure everything is functional.

Screenshots

Before:

After:

Figma Reference:

Notes

• The Figma API token was invalid, so the Figma screenshot was captured via browser instead
• Pre-existing lint errors in the codebase are unrelated to these changes

Link to Devin run: https://app.devin.ai/sessions/5ca15e50f63a41d499862b493b3a3053
Requested by: Joao Esteves (joao.esteves@cognition.ai) / @joao-cognition

[devin-ai-integration]

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