feat(ai-outfit): clear 'no pending rows' message + keep preview on generate (§19.10)

- _on_finished(total==0) now shows _show_no_pending_message: counts 完成/失败
  via read_all_rows and tells the user how to redo (clear E column / retry
  failed), instead of the silent '完成 0，失败 0'.
- _on_tasks_loaded no longer calls _fill_sample_combo(tasks), so generating on
  an all-完成 sheet no longer empties the preview dropdown (read_all_rows owns it).
- Offscreen-verified both paths; full suite (12 files) green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/widgets/ai_outfit_panel.py

@@ -806,7 +806,8 @@ class AiOutfitPanel(QWidget):
             self._set_cell(row, 5, "—")
         self._progress.setMaximum(max(1, len(tasks)))
         self._update_stats(0, 0, len(tasks))
-        self._fill_sample_combo(tasks)
+        # Preview sample rows are owned by read_all_rows (§10.3/§10.4); don't reset
+        # them to the run's task list (which is empty when the sheet is all 完成).
         self._append_log("已加载 {} 行待处理任务".format(len(tasks)))
 
     def _on_progress(self, completed, total, result):
@@ -831,12 +832,43 @@ class AiOutfitPanel(QWidget):
 
     def _on_finished(self, summary):
         self._set_running(False)
+        if getattr(summary, "total", 0) == 0:
+            self._append_log("没有待处理的行。")
+            self._show_no_pending_message()
+            return
         msg = "完成 {}，失败 {}{}".format(
             summary.success_count, summary.failure_count,
             "（已停止）" if getattr(summary, "stopped", False) else "")
         self._append_log("批量结束：" + msg)
         QMessageBox.information(self, "AI 穿搭", "本次生成结束。\n" + msg)
 
+    def _show_no_pending_message(self):
+        """Explain why nothing ran: rows already 完成/失败, and how to redo (§10.4)."""
+        done = failed = 0
+        excel = self._excel_edit.text().strip()
+        if excel:
+            try:
+                from services.excel_service import (
+                    STATUS_DONE, STATUS_FAILED, read_all_rows)
+                for r in read_all_rows(excel):
+                    if r.status == STATUS_DONE:
+                        done += 1
+                    elif r.status == STATUS_FAILED:
+                        failed += 1
+            except Exception:                            # noqa: BLE001 - best effort
+                pass
+        if done == 0 and failed == 0:
+            text = "该表没有可处理的行（标题/货号/衣服图为空的行会被跳过）。"
+        else:
+            parts = []
+            if done:
+                parts.append("已完成 {} 行（已跳过）".format(done))
+            if failed:
+                parts.append("失败 {} 行（勾选「重试上次失败的行」可重做）".format(failed))
+            text = ("该表没有待处理的行：" + "；".join(parts)
+                    + "。\n要重做已完成的行，请清空对应行的状态(E)列后重试。")
+        QMessageBox.information(self, "没有待处理的行", text)
+
     def _on_failed(self, message):
         self._set_running(False)
         self._append_log("无法开始：" + message)