feat(providers): tighten Add Provider header spacing and add footer hint

- Reduce title->tabs and tabs->card vertical gaps from 24px to 12px
- Add an always-visible footer hint guiding users to fill the fields
  below after choosing a preset (pinned footer, never scrolls off)
- FullScreenPanel: add optional contentClassName prop to scope content
  padding overrides without affecting the other panels that share it
- i18n: add provider.addFooterHint in zh/en/ja/zh-TW

src/components/providers/AddProviderDialog.tsx

@@ -280,6 +280,9 @@ export function AddProviderDialog({
   const footer =
     !showUniversalTab || activeTab === "app-specific" ? (
       <>
+        <span className="mr-auto min-w-0 text-xs text-muted-foreground truncate">
+          {t("provider.addFooterHint")}
+        </span>
         <Button
           variant="outline"
           onClick={() => onOpenChange(false)}
@@ -322,6 +325,7 @@ export function AddProviderDialog({
       title={t("provider.addNewProvider")}
       onClose={() => onOpenChange(false)}
       footer={footer}
+      contentClassName="pt-3"
     >
       {showUniversalTab ? (
         <Tabs