_flipQA regex rejects quoted <hr id="answer"> attribute, causing confusing warning

## Summary

The `_flipQA` method in `qt/aqt/clayout.py` uses a regex that only recognises the unquoted HTML form `

---

`. The equally valid — and far more common — quoted forms `

---

` and `

---

` do not match, so clicking the **Flip Q↔A** button in the card template editor silently fails and displays:

> *Anki couldn't find the line between the question and answer. Please adjust the template manually to switch the question and answer.*

This is confusing because the template is perfectly valid; the attribute is simply quoted, as any modern HTML editor or linter would produce.

## Affected code

**File:** `qt/aqt/clayout.py`

```python
def _flipQA(self, src: dict, dst: dict) -> None:
m = re.match("(?s)(.+)

---

(.+)", src["afmt"])
if not m:
showInfo(tr.card_templates_anki_couldnt_find_the_line_between())
return
self.change_tracker.mark_basic()
dst["afmt"] = "{{FrontSide}}\n\n

---

\n\n%s" % src["qfmt"]
dst["qfmt"] = m.group(2).strip()
```

The regex on line 2 requires the exact literal string `

---

`. It will not match any of these equally valid HTML spellings:

| Template text | Matches? |
|---|---|
| `

---

` | ✅ |
| `

---

` | ❌ |
| `

---

` | ❌ |
| `

---

` | ❌ |
| `

---

` | ❌ |

## Steps to reproduce

1. Create a new note type (or edit an existing one).
2. In the back template, write the separator as `

---

` (with quotes — the default produced by most HTML editors).
3. Open the card template editor and click **Flip Q↔A**.
4. Observe the warning: *"Anki couldn't find the line between the question and answer…"* — even though the template is valid.

Start in qt/aqt/clayout.py at _flipQA and reproduce the failure with the quoted separator forms listed in the issue. Verify that Flip Q↔A accepts quoted and self-closing

---

variants, preserves the existing behavior for the unquoted form, and no longer shows the warning for valid templates.

Written by the indexing model from the issue text.