Upgrade to Svelte 5

[eyeseast]

Svelte 5 is supposed to be compatible with Svelte 4, so this should just mean changing the version number in package.json, though I expect we'll run into some dependency issues.

https://svelte.dev/docs/svelte/v5-migration-guide

Migrating components to use the new runes syntax will be a longer process that we can do incrementally, though I suspect it'll ultimately make the code better. We might make a dedicated milestone for migration.