feat(profile): add help screen and fix upload field name

Add dedicated Help screen to profile tab with navigation integration.
Refactor SettingsScreen to navigate to HelpScreen instead of showing placeholder alert.
Fix API upload method to use correct field names ("file" for documents, "image" for media)
based on endpoint path, resolving upload failures for file uploads.

src/screens/profile/SettingsScreen.tsx

@@ -271,7 +271,7 @@ export const SettingsScreen: React.FC = () => {
         router.push(hrefs.hrefProfileAbout());
         break;
       case 'help':
-        Alert.alert('帮助与反馈', '帮助中心即将上线！');
+        router.push(hrefs.hrefProfileHelp());
         break;
 
       case 'edit_profile':