fix(prefs): rename default action -> savePrefs (SvelteKit named-action rule)

Vercel was returning 500 on every POST to /settings/notifications:

  Error: When using named actions, the default action cannot be used.

PR #70 added a `resendVerification` named action while keeping the
existing `default` action for the Save Changes form. SvelteKit
forbids that combination — once any named action exists on a page,
ALL forms must target a named action explicitly. The error was
swallowed by use:enhance so the user just saw "nothing happens" on
click; production logs surfaced it.

Renames `default` -> `savePrefs` and points the Save Changes form at
`?/savePrefs`. resendVerification is unchanged. Comment on the new
action calls out the constraint so a future reviewer doesn't
re-introduce a default.

Author: JWhite212

src/routes/(app)/settings/notifications/+page.server.ts

@@ -34,7 +34,12 @@ export const load: PageServerLoad = async ({ locals }) => {
 };
 
 export const actions: Actions = {
-  default: async ({ request, locals }) => {
+  // SvelteKit rejects a `default` action when ANY named action is
+  // also defined — the resendVerification action below makes this
+  // page named-only, so the Save Changes form targets `?/savePrefs`
+  // explicitly. Previously the default coexistence threw a 500 on
+  // every POST. https://svelte.dev/docs/kit/form-actions#named-actions
+  savePrefs: async ({ request, locals }) => {
     const formData = Object.fromEntries(await request.formData());
     const parsed = notificationSchema.safeParse(formData);
     if (!parsed.success) return fail(400, { errors: parsed.error.flatten().fieldErrors });

src/routes/(app)/settings/notifications/+page.svelte

@@ -144,7 +144,7 @@
   {/if}
 
   <GlassCard>
-    <form method="POST" use:enhance class="space-y-6">
+    <form method="POST" action="?/savePrefs" use:enhance class="space-y-6">
       <fieldset class="m-0 border-0 p-0">
         <legend class="text-text-muted mb-3 text-xs font-medium tracking-wider uppercase">
           Overdue dose reminders