Refine AdvancedPaste Terms UI and add default system prompt for Advanced AI (#43526)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
1. when no terms and Privacy links don't show the infobar
<img width="349" height="127" alt="image"
src="https://github.com/user-attachments/assets/b7eeeabf-365f-45f6-adb4-56335c14e8ad"
/>
<img width="410" height="217" alt="image"
src="https://github.com/user-attachments/assets/15e053c4-738d-4bb4-9544-24bdf8a5a584"
/>


2. add add default system prompt for Advanced AI

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## PR Checklist

- [ ] Closes: #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
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for new binaries
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and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/AdvancedPaste/AdvancedPaste/AdvancedPasteXAML/Pages/MainPage.xaml

@@ -264,7 +264,8 @@
                     <Button
                         Padding="0"
                         VerticalAlignment="Center"
-                        Style="{StaticResource SubtleButtonStyle}">
+                        Style="{StaticResource SubtleButtonStyle}"
+                        Visibility="{x:Bind ViewModel.HasLegalLinks, Mode=OneWay, Converter={StaticResource BoolToVisibilityConverter}}">
                         <FontIcon FontSize="12" Glyph="&#xE946;" />
                         <Button.Flyout>
                             <Flyout>

src/modules/AdvancedPaste/AdvancedPaste/Services/KernelServiceBase.cs

@@ -29,6 +29,7 @@ public abstract class KernelServiceBase(
     ICustomActionTransformService customActionTransformService) : IKernelService
 {
     private const string PromptParameterName = "prompt";
+    private const string DefaultSystemPrompt = "You are an agent who is tasked with helping users paste their clipboard data. You have functions available to help you with this task. Call function when necessary to help user finish the transformation task. You never need to ask permission, always try to do as the user asks. The user will only input one message and will not be available for further questions, so try your best. The user will put in a request to format their clipboard data and you will fulfill it. Do not output anything else besides the reformatted clipboard content.";
 
     private readonly IKernelQueryCacheService _queryCacheService = queryCacheService;
     private readonly IPromptModerationService _promptModerationService = promptModerationService;
@@ -144,7 +145,8 @@ public abstract class KernelServiceBase(
 
         ChatHistory chatHistory = [];
 
-        chatHistory.AddSystemMessage(runtimeConfig.SystemPrompt);
+        var systemPrompt = string.IsNullOrWhiteSpace(runtimeConfig.SystemPrompt) ? DefaultSystemPrompt : runtimeConfig.SystemPrompt;
+        chatHistory.AddSystemMessage(systemPrompt);
         chatHistory.AddSystemMessage($"Available clipboard formats: {await kernel.GetDataFormatsAsync()}");
         chatHistory.AddUserMessage(prompt);
 

src/modules/AdvancedPaste/AdvancedPaste/ViewModels/OptionsViewModel.cs

@@ -77,6 +77,7 @@ namespace AdvancedPaste.ViewModels
         [NotifyPropertyChangedFor(nameof(PrivacyLinkUri))]
         [NotifyPropertyChangedFor(nameof(HasTermsLink))]
         [NotifyPropertyChangedFor(nameof(HasPrivacyLink))]
+        [NotifyPropertyChangedFor(nameof(HasLegalLinks))]
         private bool _isAllowedByGPO;
 
         [ObservableProperty]
@@ -222,6 +223,8 @@ namespace AdvancedPaste.ViewModels
 
         public bool HasPrivacyLink => GetActiveProviderMetadata().HasPrivacyLink;
 
+        public bool HasLegalLinks => HasTermsLink || HasPrivacyLink;
+
         public bool ClipboardHasData => AvailableClipboardFormats != ClipboardFormat.None;
 
         public bool ClipboardHasDataForCustomAI => PasteFormat.SupportsClipboardFormats(CustomAIFormat, AvailableClipboardFormats);
@@ -367,6 +370,7 @@ namespace AdvancedPaste.ViewModels
             OnPropertyChanged(nameof(PrivacyLinkUri));
             OnPropertyChanged(nameof(HasTermsLink));
             OnPropertyChanged(nameof(HasPrivacyLink));
+            OnPropertyChanged(nameof(HasLegalLinks));
         }
 
         private void RefreshPasteFormats()