fix: Backspace/Delete deleting prompt text (root@host:~#)

Root cause: _current_line_start was set after \r\n but BEFORE the prompt
was rendered, so Backspace could erase prompt characters after reaching
the input line start.

Fix: send \x7f (DEL) to shell on Backspace and \x1b[3~ on Delete instead
of locally deleting characters. The shell's readline handles backspace
correctly - it ignores DEL at the input line start, so the prompt is
never touched. The existing \b handler in _on_data already correctly
processes the shell's \b-space-\b echo sequence.

Verified: typing 'abc' + 3 Backspace deletes input, 5 more Backspace
leaves prompt 'root@host:~# ' intact.

ui/terminal_panel.py

@@ -571,24 +571,32 @@ class TerminalPanel(QWidget):
         self._send_bytes(b"\r")
 
     def _on_backspace(self):
+        # 发 DEL(\x7f) 给 shell，由 shell 的 readline 处理退格 + 回显(\b \b)。
+        # 这样 shell 在输入行首会忽略退格，不会删除提示符 root@host:~#。
+        # 旧方案是本地 deletePreviousChar()，但 _current_line_start 在提示符
+        # 之前，导致退到输入行首后还能继续删提示符。
+        if self.chan and self._connected:
+            self._send_bytes(b"\x7f")
+            return
+        # 未连接时 fallback：本地删除
         cursor = self.term.textCursor()
         if cursor.position() <= self._current_line_start:
-            return  # 已到行首，不删
-        # 不能跨行删（保持当前行）
-        if self._is_at_line_start(cursor):
             return
         cursor.deletePreviousChar()
 
     def _on_delete(self):
+        # 发 Delete 转义序列给 shell，由 shell 处理
+        if self.chan and self._connected:
+            self._send_bytes(b"\x1b[3~")
+            return
+        # 未连接时 fallback：本地删除
         cursor = self.term.textCursor()
         if cursor.atEnd():
             return
-        # 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: