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/BoardView.vue

@@ -350,11 +350,11 @@ const gridBinds = computed(() => ({
       <template v-else>
       <template v-if="isMainBoard">
         <section v-if="pinnedNotes.length">
-          <h2 class="mb-2 text-xs font-semibold uppercase tracking-wide text-neutral-400">Pinned</h2>
+          <h2 class="mb-2 section-label">Pinned</h2>
           <NoteGrid :notes="pinnedNotes" v-bind="gridBinds" />
         </section>
         <section v-if="otherNotes.length" :class="pinnedNotes.length ? 'mt-8' : ''">
-          <h2 v-if="pinnedNotes.length" class="mb-2 text-xs font-semibold uppercase tracking-wide text-neutral-400">
+          <h2 v-if="pinnedNotes.length" class="mb-2 section-label">
             Others
           </h2>
           <NoteGrid :notes="otherNotes" v-bind="gridBinds" />