feat: the docs header links the console and shows GitHub as an icon

[robbeverhelst]

The docs header now looks like the landing page's (sluiceway/landing #25), following the design system. Go to console (https://console.sluiceway.dev) is the one thing in the bar that looks pressable. The GitHub and Docs repo text links are replaced by one small quiet GitHub mark that links https://github.com/sluiceway/sluiceway. It has the aria-label "Sluiceway on GitHub", no label text, and no outline or shadow.

• Header override: the button sits beside search below 50rem, so it stays in the bar on a phone, and in the right group from 50rem up. It is the design system's default button at sm, the same as the landing page.
• SocialIcons: the Sluiceway text link, then the GitHub mark (Octicons mark-github-16 from @primer/octicons 19.38.0). Its hit area is 36px, or 44px on a touch screen.
• Quieter controls, so the console button is the only thing with weight: the theme switch has no outline or shadow and is ink-muted, as on the landing page. Search and the phone menu button keep their outline and lose their shadow. The divider after the links is gone.
• At phone width, below 500px the wordmark is read out but not shown, so the mark, search, Go to console and Menu fit at 320px. The menu button is sized through --sl-menu-button-size, so it sits level with the rest of the bar (before, it sat 8px low). The menu's last row, with the links, the GitHub mark and the theme switch, is on one line.
• Between 50rem and 60rem, search shows as its icon. Without that, the theme switch was cut off at 800px.
• The docs repo is still linked from every page's "Source:" line. The console link is counted by the existing outbound event, so nothing in analytics changes.

Screenshots

Desktop, 1280px

	Light	Dark
Before
After

Phone, 360px and 320px

	Before 360	After 360	After 320
Light
Dark

The phone menu, scrolled to its last row

	Before 360	After 360	After 320
Light
Dark

Checks

• bun run check passes: lint, tokens, pin, release, typecheck, build, 617 tests.
• bun run links: 33003 links on 150 pages, all of them land.
• Lighthouse (built at /): 98–99 performance and 100 on everything else, except /what-it-looks-like/ at 96 accessibility. That comes from two <summary> touch targets in the page content, not the header, and is above the 0.9 threshold.
• No horizontal scroll at 320, 360, 800, 850, 900, 959, 961 or 1024px on a page whose content fits.

Not in this PR

Get started overflows to 362px on a phone: the inline code .github/workflows/deploy-dashboard.yml never wraps (theme.css: "Stack ids never wrap"). That was already the case before this PR. At 320px it pushes the fixed menu button partly off-screen on that page.

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