feat: match the docs visual redesign

[easymac]

Ports the visual redesign from the docs site (shorebirdtech/docs #500, #504, and the header logo size fix) to the handbook. No content files are touched.

What changed

• Swap @astrojs/starlight-tailwind for starlight-theme-nova, and set expressiveCode: false as Nova requires.
• Replace src/tailwind.css with src/styles/custom.css, copied from docs: General Sans font faces, background colors, restored list bullets, dark-mode code block background, nested sidebar label styling, and the 1.5rem header logo.
• Add the General Sans woff2 files under public/fonts/ and a font preload in the head.
• Add the animated ThemeSelect.astro override and its two Tabler icons from docs.

Skipped from docs on purpose: the MarkdownContent and Head overrides only exist for image zoom and OG image generation, which the handbook does not use. The docs content changes (LinkCard, CodeTabs, LinkButton) are not ported because they rewrite copy.

One intentional visual change: the old teal accent palette is gone, so links render as underlined text in Nova's neutral palette, matching docs.

Verification

• Built the site before and after and diffed the rendered text of all 50 pages. Markdown content is identical on all pages. The only differences are Nova dropping the "Previous"/"Next" words from pagination links and the old code renderer's "Terminal window" frame label on the onboarding code block.
• Checked both themes on desktop and a 390px mobile layout: the compliance page's light/dark ISO badge swap still works (Nova sets data-theme as well as the dark class), the FSQS logo and CEO headshot render, code blocks, lists, tables, and the note aside render, the three-level nested sidebar renders, search returns Pagefind results, and the mobile menu opens.
• npm run format:check, npx --no allow-scripts, and npm run build (astro check plus build with link validation) all pass locally.