fix: Delete key in terminal deleting prompt text (root@host)

_on_delete() was missing _current_line_start check that _on_backspace
already had. Clicking into the prompt area (root@hostname:~#) and pressing
Delete would erase prompt characters. Now:
- Cursor before input line: moved to input start, no deletion
- Selection crossing prompt boundary: blocked

ui/terminal_panel.py

@@ -583,7 +583,15 @@ class TerminalPanel(QWidget):
         cursor = self.term.textCursor()
         if cursor.atEnd():
             return
-        if cursor.columnNumber() == 0 and not cursor.hasSelection():
+        # 不允许删除当前输入行之前的内容（提示符 root@host:~# 等）
+        if cursor.position() < self._current_line_start:
+            cursor.setPosition(self._current_line_start)
+            self.term.setTextCursor(cursor)
+            return
+        # 如果有选区且选区起点在当前输入行之前，不允许
+        if cursor.hasSelection():
+            sel_start = cursor.selectionStart()
+            if sel_start < self._current_line_start:
                 return
         cursor.deleteChar()