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/common/FullScreenPanel.tsx

@@ -10,6 +10,7 @@ import {
   DRAG_REGION_STYLE,
 } from "@/lib/platform";
 import { isTextEditableTarget } from "@/utils/domUtils";
+import { cn } from "@/lib/utils";
 
 interface FullScreenPanelProps {
   isOpen: boolean;
@@ -17,6 +18,11 @@ interface FullScreenPanelProps {
   onClose: () => void;
   children: React.ReactNode;
   footer?: React.ReactNode;
+  /**
+   * 覆盖内容区滚动容器的内边距/间距类。默认 `px-6 py-6 space-y-6`。
+   * 通过 `cn`(twMerge) 合并，传入如 `pt-3` 只覆盖顶部内边距，其余保持默认。
+   */
+  contentClassName?: string;
 }
 
 const DRAG_BAR_HEIGHT = isWindows() || isLinux() ? 0 : 28; // px - match App.tsx
@@ -33,6 +39,7 @@ export const FullScreenPanel: React.FC<FullScreenPanelProps> = ({
   onClose,
   children,
   footer,
+  contentClassName,
 }) => {
   React.useEffect(() => {
     if (isOpen) {
@@ -136,7 +143,9 @@ export const FullScreenPanel: React.FC<FullScreenPanelProps> = ({
 
           {/* Content */}
           <div className="flex-1 overflow-y-auto scroll-overlay">
-            <div className="px-6 py-6 space-y-6 w-full">{children}</div>
+            <div className={cn("px-6 py-6 space-y-6 w-full", contentClassName)}>
+              {children}
+            </div>
           </div>
 
           {/* Footer */}

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