GUI/QValidatedLineEdit: add validation control features and fix error highlighting

  - Added setAllowEmptyInput() to control whether empty input is considered valid
  - Added setAllowValidationWhileEditing() to enable real-time validation while typing
  - Fixed error_locations handling to only apply character-level formatting for validators that support it (BitcoinAddressEntryValidator), preventing incorrect highlighting of first character for standard validators
  - When validation while editing is enabled, markValid() now triggers checkValidity() instead of forcing valid state

Author: bigshiny90

src/qt/qvalidatedlineedit.cpp

@@ -92,8 +92,10 @@ void QValidatedLineEdit::setValid(bool _valid, bool with_warning, const std::vec
 
 void QValidatedLineEdit::focusInEvent(QFocusEvent *evt)
 {
-    // Clear invalid flag on focus
-    setValid(true);
+    // Clear invalid flag on focus, unless we're validating while editing
+    if (!m_allow_validation_while_editing) {
+        setValid(true);
+    }
 
     QLineEdit::focusInEvent(evt);
 }
@@ -108,7 +110,12 @@ void QValidatedLineEdit::focusOutEvent(QFocusEvent *evt)
 void QValidatedLineEdit::markValid()
 {
     // As long as a user is typing ensure we display state as valid
-    setValid(true);
+    // unless we're validating while editing
+    if (m_allow_validation_while_editing) {
+        checkValidity();
+    } else {
+        setValid(true);
+    }
 }
 
 void QValidatedLineEdit::clear()
@@ -138,7 +145,7 @@ void QValidatedLineEdit::checkValidity()
     const bool has_warning = checkWarning();
     if (text().isEmpty())
     {
-        setValid(true);
+        setValid(m_allow_empty_input);
     }
     else if (hasAcceptableInput())
     {
@@ -156,7 +163,7 @@ void QValidatedLineEdit::checkValidity()
             } else {
                 int pos = 0;
                 validation_result = checkValidator->validate(address, pos);
-                error_locations.push_back(pos);
+                // do not provide error locations for validators that do not support it
             }
             if (validation_result == QValidator::Acceptable)
                 setValid(true, has_warning);

src/qt/qvalidatedlineedit.h

@@ -22,6 +22,8 @@ class QValidatedLineEdit : public QLineEdit
     bool isValid();
     void setWarningValidator(const QValidator *);
     bool hasWarning() const;
+    void setAllowEmptyInput(bool allow) { m_allow_empty_input = allow; }
+    void setAllowValidationWhileEditing(bool allow) { m_allow_validation_while_editing = allow; }
 
 protected:
     void focusInEvent(QFocusEvent *evt) override;
@@ -32,6 +34,8 @@ class QValidatedLineEdit : public QLineEdit
     const QValidator* checkValidator{nullptr};
     bool m_has_warning{false};
     const QValidator *m_warning_validator{nullptr};
+    bool m_allow_empty_input{true};
+    bool m_allow_validation_while_editing{false};
 
 public Q_SLOTS:
     void setText(const QString&);