feat(web): theme/locale preferences and improve SSR

[michelheusschen]

Creating a new PR from #1814, because that one had a bit of a snowball effect.

• Revamped theme switching in general and added a store for managing preferences
• Added the svelte-local-storage-store package for persisting preferences to localStorage
• Fixed Flash of Unstyled Content (FOUC) on initial page load when using the light theme

Suddenly SSR started actually working. Previously any server-side rendered page would have an empty <main></main> element so it wasn't doing much. Having SSR for pages means we can't use document, navigator, window, etc and a couple of issues popped up because of that:

• Can no longer use navigator.language, so moved locale to preferences store
• Jest can't import $app/environment so a manual mock has been added
• Guarded browser-only calls in the lifecycle function onDestroy, because it also runs for SSR

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1 Ignored Deployment

Name	Status	Preview	Updated
immich	⬜️ Ignored (Inspect)		Feb 22, 2023 at 1:32PM (UTC)