refactor(settings/providers): align with DS conventions

Two consistency wins from the screenshot/DS audit pass.

Sync icon button now renders DS::Button (variant: icon, size: sm)
instead of a hand-rolled `button_to`. Same component used by other
icon-only actions across the app (settings/profiles, layouts/imports).
Visual delta: 28×28 → 32×32 (DS sm size). Accept the +4px for
consistency. `event.stopPropagation()` still wired via the form opt
so the row's <details> doesn't toggle when the user clicks the
button.

Group heading now follows the established Sure section-label style
(`text-xs font-medium text-secondary uppercase`) used by
`_settings_nav` and the imports/categories surfaces. The previous
sentence-case `text-sm text-primary` was a one-off that didn't
match the rest of the app. Locale strings stay sentence-case;
uppercase comes from CSS `text-transform`. Tests updated to
case-insensitively match the rendered heading text.

app/views/settings/providers/_sync_button.html.erb

@@ -1,12 +1,15 @@
 <%# locals: (provider_key:, last_synced_at: nil) %>
 <% recently_synced = last_synced_at.present? && last_synced_at > 60.seconds.ago %>
 <% button_label = recently_synced ? t("settings.providers.recently_synced") : t("settings.providers.sync_provider") %>
-<%= button_to sync_provider_settings_providers_path(provider_key: provider_key),
-              method: :post,
-              disabled: recently_synced,
-              title: button_label,
-              aria: { label: button_label },
-              class: "inline-flex items-center justify-center w-7 h-7 rounded-md text-secondary hover:text-primary hover:bg-alpha-black-50 transition-colors disabled:opacity-40 disabled:cursor-not-allowed",
-              form: { onclick: "event.stopPropagation()", class: "inline-flex" } do %>
-  <%= icon "refresh-cw", size: "sm", class: "!w-3.5 !h-3.5" %>
-<% end %>
+<%= render DS::Button.new(
+      variant: "icon",
+      size: "sm",
+      icon: "refresh-cw",
+      href: sync_provider_settings_providers_path(provider_key: provider_key),
+      method: :post,
+      disabled: recently_synced,
+      title: button_label,
+      aria: { label: button_label },
+      class: "disabled:opacity-40 disabled:cursor-not-allowed",
+      form: { onclick: "event.stopPropagation()", class: "inline-flex" }
+    ) %>