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/stores/toasts.svelte.ts

@@ -0,0 +1,35 @@
+let nextId = 0;
+
+export type ToastType = 'success' | 'error' | 'warning' | 'info';
+
+export type Toast = {
+	id: number;
+	type: ToastType;
+	message: string;
+};
+
+let toasts = $state<Toast[]>([]);
+
+export function getToasts(): Toast[] {
+	return toasts;
+}
+
+export function addToast(type: ToastType, message: string, duration = 4000): number {
+	const id = nextId++;
+	toasts.push({ id, type, message });
+	if (duration > 0) {
+		setTimeout(() => removeToast(id), duration);
+	}
+	return id;
+}
+
+export function removeToast(id: number) {
+	toasts = toasts.filter((t) => t.id !== id);
+}
+
+export const toast = {
+	success: (msg: string, duration?: number) => addToast('success', msg, duration),
+	error: (msg: string, duration?: number) => addToast('error', msg, duration ?? 6000),
+	warning: (msg: string, duration?: number) => addToast('warning', msg, duration),
+	info: (msg: string, duration?: number) => addToast('info', msg, duration)
+};