Fix color mode: only persist explicit choice, follow system theme

[Edward-Deakin]

Problem

Auto theme switching defaulted to a stale value in `localStorage`. The provider wrote the resolved mode to `localStorage.theme` on every load, so the system `prefers-color-scheme` default (or a `?docusaurus-theme=` URL param) became a stored "choice" that overrode later system changes.

Fix (`packages/docs-ui/src/providers/ColorMode`)

• Persist to `localStorage` only on explicit user set/toggle
• Priority on load: URL param (this view only, not persisted) → stored explicit choice → system preference
• Follow live `prefers-color-scheme` changes while no explicit choice is stored
• Guard storage access with try/catch; validate stored values

Notes

• Users who already have a `theme` value saved from the old behaviour keep it until they toggle or clear it.
• Verified: `tsc --noEmit` clean, Prettier clean. ESLint still reports only quote/semicolon rule conflicts that also fire on the untouched file (ESLint config expects double quotes; repo Prettier uses single).
• Not tested in a browser.

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