DRY pass #2, batch 5, F14: the web's repeated class lists become component classes (#5372)

style.css gains the classes the views spelled out in full: .section-label
(17 sites), .hint (20), .form-error (13), .alert-error (5), .row-card (5),
.list-empty (5), .field (5), .page-shell (3), and the small row action
.btn-sm (4) / .btn-sm-danger (3). Only exact runs moved, so nothing renders
differently; spacing a site adds beyond a run stays a utility beside it.

Kept: the Reminders and Timeline small buttons. They carry no text colour
and inherit it, so putting them on .btn-sm would recolour them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/views/RegisterView.vue

@@ -52,7 +52,7 @@ async function submit() {
           height="48"
         />
         <h1 class="text-2xl font-bold tracking-tight">{{ invite ? "You're invited" : "Create your space" }}</h1>
-        <p class="mt-1 text-sm text-neutral-500 dark:text-neutral-400">
+        <p class="mt-1 hint">
           {{ invite ? `Create your account on ${config.siteName}.` : "Start capturing in seconds." }}
         </p>
       </div>
@@ -86,14 +86,14 @@ async function submit() {
         <p
           v-if="error"
           role="alert"
-          class="rounded-lg bg-red-50 px-3 py-2 text-sm text-red-700 dark:bg-red-950/50 dark:text-red-300"
+          class="alert-error"
         >
           {{ error }}
         </p>
         <BaseButton type="submit" :loading="loading">Create account</BaseButton>
       </form>
 
-      <p class="mt-6 text-center text-sm text-neutral-500 dark:text-neutral-400">
+      <p class="mt-6 text-center hint">
         Already have an account?
         <RouterLink to="/login" class="font-semibold text-brand-700 hover:underline dark:text-brand"
           >Sign in</RouterLink