Finish Svelte 5 migration

Some components still use the legacy export let pattern instead of the Svelte 5 $props() rune.

Components to Migrate

• src/lib/Accordion.svelte- uses export let
• src/lib/AnchorHeading.svelte - uses export let
• src/lib/PublicationAuthors.svelte - uses export let (3 props)
• src/lib/Fun/Timer.svelte- uses export let

Deprecated Imports

• src/routes/+error.svelte - uses $app/stores instead of $app/state — migrated
• src/routes/Header.svelte - uses $app/stores instead of $app/state