feat: white-label branding support

[pthmas]

Summary

• Add runtime-configurable branding via 7 optional env vars (CHAIN_NAME, CHAIN_LOGO_URL, ACCENT_COLOR, BACKGROUND_COLOR_DARK, BACKGROUND_COLOR_LIGHT, SUCCESS_COLOR, ERROR_COLOR)
• New backend GET /api/config endpoint serves branding config from environment variables
• Frontend BrandingContext fetches config once on load, applies CSS custom properties for accent colors and derives surface palettes from background colors
• Logo, chain name, favicon, and page title update dynamically
• All values optional — defaults match current Atlas branding when unset

Test plan

• Set custom CHAIN_NAME and ACCENT_COLOR in .env, verify title/links/buttons change
• Set BACKGROUND_COLOR_DARK / BACKGROUND_COLOR_LIGHT, verify surface colors adapt in both themes
• Set CHAIN_LOGO_URL pointing to mounted branding asset, verify logo in navbar and welcome page
• Verify with no branding env vars set, UI looks identical to current Atlas
• Test dark/light theme toggle with custom backgrounds
• Verify bunx vite build succeeds
• Verify docker compose build && docker compose up -d works end-to-end

🤖 Generated with Claude Code

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch pierrick/white-labeling

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