migrate: Update to Svelte 5

[Xyphyn]

Quite a big task. Here's what needs to happen:

• [x] Make app build successfully
• [ ] Fix all the errors
• [ ] Update logic to take advantage of Svelte 5's features

Other minor tasks just so I remember them:

• [ ] Change stores to states, thanks to the new svelte.ts ending
• [ ] Take advantage of snippets in some places
• [ ] Fix post bodies not being considered expanded even when not overflowing

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