Fix $state runtime error: rename rune stores to .svelte.ts

$state runes only work in .svelte and .svelte.ts files. The toasts and
theme stores used $state in plain .ts files, which compiled in dev but
crashed in production SSR. Renamed and updated all imports.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/components/Toast.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { fly } from 'svelte/transition';
-	import { getToasts, removeToast, type ToastType } from '$lib/stores/toasts.js';
+	import { getToasts, removeToast, type ToastType } from '$lib/stores/toasts.svelte.js';
 
 	const toasts = $derived(getToasts());