fix: improve documentation contrast and site titles

[nick-csu]

Changes

• Calculate primary button text contrast from sRGB luminance of the configured fill.
• Keep the button fill consistent in light and dark themes; adjust standalone accents separately.
• Share the CSS generator between server rendering and live homepage configuration updates.
• Add regression coverage for dark and light fills, including the three hosted documentation demo colors.

Verification

• Docs UI unit tests: 30 passed.
• Repository lint: passed.
• Static browser regression cases cover orange, blue, purple, and white fills in both appearances.
• Redeploying the three documentation examples and checking computed button colors through Playwright MCP.

Site titles

The documentation header now uses the configured display title before the hosting project slug, so sites can use a readable name while keeping their reserved subdomains.