Fix navbar logo offset on wide screens

[maddy-abrahams]

Summary

• Decouples SiteHeader's logo/nav row from the page content's max-width container. Previously the header centered inside the same max-width box as the content below it, so on wide monitors the logo sat hundreds of pixels from the left edge instead of hugging it.
• SiteHeader now uses fixed padding only (px-4 sm:px-6 lg:px-8), so the logo stays a constant distance from the viewport edge at any screen width.
• Removed the now-unused contentMaxWidth prop from SiteHeader and updated all callers (standard layout, analysis layout, experiments/ndeltas page, experiments/ndeltas/[modelId] page).
• Applied the same fix to the /about page's own header: dropped its max-width container so the logo hugs the left edge, matching the rest of the site.

Test plan

• Load / and resize the browser from mobile width up to an ultra-wide monitor width -- confirm the logo stays anchored near the left edge throughout, with no large gap appearing on wide screens.
• Check /analysis/[configId], /experiments/ndeltas, and /experiments/ndeltas/[modelId] -- header should still render correctly (no longer width-matched to that page's wider content grid, which is expected.
• Check /about -- header logo should sit at the same left-edge position as the home header.

🤖 Generated with Claude Code
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