docs: add FOUC prevention guide for CSR/SPA applications

[braden-w]

Motivation

Users of mode-watcher in CSR/SPA contexts (SvelteKit with ssr: false, Vite SPAs, static sites) can experience FOUC because JavaScript runs after initial paint—unless you add a script manually into the head tag of app.html.

The existing createInitialModeExpression utility relies on server hooks, which don't run when serving prerendered or static pages. This guide documents the recommended inline script pattern for those cases.

What's Included

• New "Guides" section in the navigation
• guides/csr-spa-fouc.md covering the inline script solution
• Cross-links from Getting Started and createInitialModeExpression pages

Why Not CSS-Only?

CSS media queries (prefers-color-scheme) only see OS preference—not the user's stored choice in localStorage. If a user chose dark mode but their OS is light, they still get FOUC.

References

• sveltejs/kit#13307 — Detailed technical explanation
• EpicenterHQ/epicenter#1168 — Real-world implementation
• Past mode-watcher issues: #110, #111

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