Set the mention and error color according to colorscheme

Use kirigami to set the proper link and error color for mentions and error underlining in chatbox

This is with my own colorscheme:

![image](/uploads/6269b85f936a98b02f42a9ce01da76b1/image.png)

src/chatdocumenthandler.h

@@ -35,6 +35,9 @@ class ChatDocumentHandler : public QObject
 
     Q_PROPERTY(NeoChatRoom *room READ room WRITE setRoom NOTIFY roomChanged)
 
+    Q_PROPERTY(QColor mentionColor READ mentionColor WRITE setMentionColor NOTIFY mentionColorChanged);
+    Q_PROPERTY(QColor errorColor READ errorColor WRITE setErrorColor NOTIFY errorColorChanged);
+
 public:
     explicit ChatDocumentHandler(QObject *parent = nullptr);
 
@@ -60,6 +63,13 @@ public:
 
     void updateCompletions();
     CompletionModel *completionModel() const;
+
+    [[nodiscard]] QColor mentionColor() const;
+    void setMentionColor(const QColor &color);
+
+    [[nodiscard]] QColor errorColor() const;
+    void setErrorColor(const QColor &color);
+
 Q_SIGNALS:
     void isEditChanged();
     void documentChanged();
@@ -68,6 +78,8 @@ Q_SIGNALS:
     void completionModelChanged();
     void selectionStartChanged();
     void selectionEndChanged();
+    void errorColorChanged();
+    void mentionColorChanged();
 
 private:
     int completionStartIndex() const;
@@ -79,6 +91,9 @@ private:
     NeoChatRoom *m_room = nullptr;
     bool completionVisible = false;
 
+    QColor m_mentionColor;
+    QColor m_errorColor;
+
     int m_cursorPosition;
     int m_selectionStart;
     int m_selectionEnd;