fix: add trailing slash to preferences API calls in portal (#362)

the /preferences endpoint has a trailing slash, so requests without it
get 307 redirected. behind fly's proxy, this redirect was generating
http:// URLs causing mixed content errors. matching the pattern already
used in SettingsMenu.svelte.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[e-mail]>

frontend/src/routes/portal/+page.svelte

···
 
 	async function loadPreferences() {
 		try {
-			const response = await fetch(`${API_URL}/preferences`, {
+			const response = await fetch(`${API_URL}/preferences/`, {
 				credentials: 'include'
 			});
 			if (response.ok) {
···
 
 	async function saveAllowComments(enabled: boolean) {
 		try {
-			const response = await fetch(`${API_URL}/preferences`, {
+			const response = await fetch(`${API_URL}/preferences/`, {
 				method: 'POST',
 				headers: { 'Content-Type': 'application/json' },
 				credentials: 'include',